Generally, when you buy a car at the bottom end of the market, you can consider yourself lucky if it lasts longer than the expiry date on the MoT. Rust is the big killer, eating away at the vital structural parts of your car until the bodyshell is so weak that you start taking a twenty mile detour to avoid speed bumps. If the rust doesn't kill your old banger, then fifteen years of total neglect, twenty thousand mile service intervals and driving two hundred yards to the shops with the choke out will most likely wear out the mechanical parts, so that repairs become more expensive than the car is worth.
However, some cars are tougher than others, and by choosing carefully you stand a good chance of buying a car that will just keep going on and on. Just remember, if you want your car to last, it needs to be in decent nick when you buy it, and you need to spend a bit of time and money to keep it that way. Change the oil and filter every 6,000 miles, clean out the mud from underneath the wheelarches once a month, and don't thrash the engine, especially from cold.
Here is the Bangernomics guide to the cheap cars most likely to hit the 20 year / 200,000 mile barrier. No warranties given or implied, so if you go out and buy a VW Golf and it falls to bits after three months, don't bother suing me - I haven't got any money anyway.
Power steering, alloys, electric windows, central locking and a gorgeous five cylinder engine - all for six hundred quid.
These are quality cars and built to last, with decent rustproofing, nice reliable German electrics* and more or less unbreakable VW engines. Comfortable and usually well equipped - even the base 80 usually comes with sunroof and central locking. 80 Sport has Golf GTI engine, uprated suspension and lots of kit. Audi 90 is basically an 80 with bigger engine and luxury fittings. Insurance a bit higher than Sierra/Cavalier, but cheap parts through specialists such as German & Swedish. Series 2 cars (1988 on) now getting very cheap, but trickier to work on. Most likely cause of death - cambelt failure (change every 60,000 miles), rust in sills and rear suspension mounting points.
* Some Audi owners may disagree with this. Having spent half a day sorting out the tail lights on a series 2 Coupe I can see their point. Audis aren't perfect, but compared to Fiats, Citroens, Vauxhalls, or anything with Lucas electrics, they are a paragon of reliability.
Ford Fiesta Mk1, 1977-83 Just look at how many of these cars are still around, compared to anything else from the same era. The mechanicals are as tough as they come - this was Ford's first front-wheel drive car, and over-engineered to ensure reliability. Bodywork actually seems less rust-prone than the later cars, so there are plenty of examples out there still with the original sills and floor. Simple, boxy body shape makes welded repairs easy. Most likely cause of death - rust around front suspension mounts.
Renault 9/11, 1983-89 One of the dullest cars ever made (despite winning Car of the Year in 1984). But looks like a good bet as cheap family transport. Renault did a proper job rustproofing this one (unlike the Renault 5) and the 1400 engine is noisy but durable. Electrics can give problems with age, and the interior does not last as well as the other bits. Most likely cause of death - electrical fire, total mechanical neglect.
VW Golf Mk2, 1983-90 A Golf, well looked after, should last indefinitely. Same 1600 and 1800 engines and rustproofing as the Audi 80, durable though austere interiors, and parts are easy to get hold of (and unlikely to be needed). The 1300 version is a bit underpowered and the engine tends to have a shorter life. 1050 is a bad joke. GTIs great fun, but often stolen, thrashed and/or crashed - approach with caution. 1800GL probably the best all rounder. Most likely cause of death - colliding with another Golf.
VW Polo, 1982-90 Bank-vault build quality, decent German electrics and almost unburstable mechanicals. Mark 2 Polos rust very slowly indeed, and there are plenty of early 1980s examples still out there with nothing more than the odd stonechip to spoil their eighteen year old factory paintwork. 1050 is tediously slow, but much easier to find than 1300. Interiors are spartan but everlasting. Look out for major engine wear (the air filter will be soaked in engine oil, but click here for a fix that may solve this problem) and don't expect sharp brakes or nimble handling. Most likely cause of death - worn out engine.
Volvo 240/244/245, 1975-90 Built like a tank, and a firm favourite with the antiques trade. A true workhorse, capable of intergalactic mileages if looked after, but most examples you will see are now getting very old. Bodywork usually resists rust well, but some late Seventies examples seem to have been made of poor quality steel, so beware. Estates tend to rot out around the tailgate. Avoid rattly engines and electrical problems, and bear in mind that parts for these cars are not especially cheap. Later 740/760 series cars are now turning up in the sub-£1000 price range. Most likely cause of death - lots of minor faults which will cost too much to fix.
Saab 900, 1980-94 Those Swedes really know how to build a solid car. Definitely more fun to drive than a Volvo 240, but mechanically more complex - tend to eat steering racks, gearboxes (manual AND auto), fuel relays and pumps (like most Bosch injection equipped cars). Lots of quirky design features (wrap around windscreen, ignition lock next to gear lever), all of which make perfect sense once you get used to the Saab way of doing things. Base models are slow (especially automatics), and Turbo really needs a full service history to be worth the risk. Cooling system needs to be kept in good order. Hefty insurance premiums and costly parts, but a 900 should hold its value well thanks to minor cult status among London trendies. Saab 9000 is bigger, more expensive to run, but shares many of the same virtues. Most likely cause of death - seized engine due to overheating.
Nissan Bluebird, 1984-89 Minicabs-R-Us. A firm favourite for many years with people who earn a living from their cars, due to its mechanical toughness, impeccable reliability and capacity for huge mileages. Bodywork rusts through in the end, but no other real problems to report. Light and easy to drive, well equipped, but don't expect it to go or handle like a sports car. Ride was a bit jiggly even when new, and unlikely to have improved with age. Most likely cause of death - intergalactic mileage.
Note that I have not mentioned Mercedes or BMW. This is not because the cars aren't tough, but because they hold their value so well that the only cars turning up under a grand are likely to be close to death, and any mechanical failure more complex than a broken fanbelt is going to cost big money to fix. Still, late Seventies/early Eighties examples turn up in timewarp condition from time to time. Look for few owners and a stamped up service history, and don't assume that prestige German cars are immune from rust - they aren't.
Postscript - reader Paul Sherwin sent in the following useful information. Thanks for that, Paul:
"I think 1980s BMWs make excellent bangers, especially the E30 3 Series. The 6 cylinder ones do tend to be expensive, being popular with the Max Power brigade, but the 4 cylinder 316 and 318s are easily available for £500-£750, often having being owned by middle class families as second cars. Despite their upmarket image, these cars are very conservatively (even crudely) engineered - they weren't sneered at as 'The Bavarian Cortina' for nothing. The 4 cyl engines also use timing chains which saves changing the belt. They're fairly easy for a DIY mechanic. The 'Oil Service' light will come on after a while, but this can be safely ignored - just change the oil etc. at appropriate intervals as with any other car. Your friendly man round the corner probably has the tool to reset the service indicator if this bothers you, but it's only a nag light.
"The carburetor models had problems when new, but these are likely to have been sorted out in a banger, often by a previous owner replacing the factory carb with a Weber manual choke conversion. The engines last forever if not neglected or thrashed. The gearboxes often develop a rattle in neutral when warm, but soldier on for many years in this state.
"BMW original parts are certainly expensive, but the solution to that is obvious! Here's an example. My 1987 316 became difficult to start. I eventually decided that this was caused by a worn rotor arm, which has a gizmo on the top designed to stop you over-revving the engine. The BMW replacement was £80! I fitted a Motolec arm for £2 (without the limiter device) and the car runs perfectly.
"BMWs are starting to appear in large numbers in scrapyards now - they are killed by rust, or engine management problems on the fuel injected models, or the suspension gives up after astronomical mileages. The biggest irritant for the old BMW owner is the worn and ripped seats, which give up after about 100,000 miles, but they remain comfortable and look OK with a pair of seat covers. Scrapyard BMWs usually have knackered seats also.
"PS. One of your correspondents describes doing a mega mileage in a Mk2 Cavalier. I can confirm these cars can cover very high mileages - a friend has covered over 150,000 miles in his 1990 Cav 2 litre over the last 5 years, with only routine servicing costs - oil, brake pads, cam belts etc. He does run it on fully synthetic oil though. It's now done over 230,000 miles and still runs very well."