Ford Fiesta, Orion and Escort started out with a 1600cc engine, later increased to 1800cc. This is a sluggish motor but very tough - we have an Escort at work that has done 250,000 miles and still runs sweetly. These engines are notorious for breaking cambelts - sometimes at lower mileages than Ford's recommended service life. So change the belt 10,000 miles before it is due, just to be safe. Parts are fairly cheap and widely available, lots of cars to choose from. Rust is the biggest problem - otherwise a good dependable choice, although they seem to eat steering racks.
Ford Sierra used a 2.3 Peugeot diesel. Not exactly overpowered with 67 bhp, but a popular minicab choice due to low running costs. Estate is a useful workhorse. Most Sierras now have huge mileages under their wheels and are rotting badly, but a good one is worth considering.
Austin Maestro and Montego pioneered direct injection in small diesel cars with a 2.0 motor developed by Perkins, and also used in the Sherpa van. Direct injection makes for easier cold starting, better performance and economy, but is very noisy. These engines always sound as though they are about to blow up, and can be very wearing on a long journey. Montego had turbo motor, Maestro was non-turbo. All the usual BL problems - rust (everywhere), disintegrating interiors, shoddy electrics and zero image. But they're cheap.
Citroen BX is probably the most widely available dirt-cheap diesel. The complex hydraulic suspension puts people off, but most problems are due to rusty and damaged pipes, and fairly cheap to fix. A regular ticking from under the bonnet suggests problems, as does a car that rapidly settles onto its suspension when the engine is switched off, doesn't sit level or rides badly. Interiors quickly look shabby, and all the external plastic trim fades, so most old BXs look well overdue for the knacker's yard, but the engines were state of the art at the time, and still pretty good today. The estate is a handy bit of kit (thanks to self-levelling suspension) and with tatty but usable cars starting at £200, you can afford to treat them as disposable. Electrics can be troublesome.
Citroen AX has a 1400cc diesel. Reasonable performance in such a light car, but the 1.0 petrol goes just as well, needs less servicing, costs less to buy and still does 50 mpg, so the diesel seems a bit pointless as a banger buy.
Fiat Uno poses the same problem as the AX - why buy a diesel supermini when the petrol equivalent is almost as economical and nicer to drive? Fiat never provided a convincing answer to this question, so there aren't many diesel Unos about. Those that remain are rusting fast.
Fiat Tipo is a sort of Italian Austin Maestro - roomy but frumpy-looking bodyshell and suspect build quality. Feeble non-turbo and decently quick 92 bhp turbo available - both now very cheap. Bodyshell is allegedly galvanised steel, but it is Italian galvanising, and I have seen early examples with crumbling sills. Worth a look as cheap family transport, but perfect reliability is unlikely.
Peugeot 205 and 309 have excellent engines (same as Citroen BX) in a smart but slightly fragile package. Poor quality paint, flimsy interiors and cheap electrics, but they go and handle well, and there are plenty about. A few of the earlier 305 model are still around - these ride beautifully, and the estate has a decent load area for the size of car. Last ones made 1988, but still worth a punt for comfortable economy transport.
Peugeot 405 was a favourite with fleet buyers, so plenty to choose from. Roomy, good looking and comfortable, but the interior always seemed one size too small for the bodyshell - they all squeak and rattle. Check all the electrics work.
Renault 5 / 9 / 11 competed head-on with Peugeot's range of small diesels. Not quite as refined, and can suffer from head gasket problems which are difficult to cure. 5 rusts badly, 9 and 11 are more durable but lack sex appeal to say the least. Why can't the French make reliable electrics, or seat covers that last beyond 75,000 miles?
Renault 21 was never as popular as Peugeot 405, but not a bad old thing, especially the Savanna estates which have a huge load area and often come with seven seats. Paint falls off, electrics pack up, interior trim disintegrates, but they keep going. Don't pay too much.
Vauxhall Nova was available from late 1989 with a 1.5 litre Isuzu turbodiesel. These go well and are capable of big mileages, but are hard to find. Harsh ride, nasty dash design and lots of rust potential. Engine spares can be hard to source at a sensible price.
Vauxhall Astra / Cavalier used a home-grown diesel motor which has the same camshaft problems as its petrol cousin. Listen for top end rattles. Many cars now very rusty (especially around wheelarches) and poor quality wiring can lead to fires. Plenty about, but Ford offerings probably better value. Don't be put off by a worn clutch - you can change it yourself in under an hour with a couple of spanners and £10 worth of special tools.
VW Golf / Jetta look like a good long-term bet, with usual VW durability. But the non-turbo diesel dates back an awful long way in diesel development terms, and 55 bhp isn't a lot to play with. Later turbo cars unlikely to turn up at a banger price. Long-lasting interiors and good residual values make for a clocker's delight, so don't pay over the odds for an allegedly low mileage car unless there is a full history to back it up.
VW Passat uses the same 55 bhp diesel. Roomy, well built family car, with potential for big mileages, but absurdly underpowered. Just imagine an estate, fully laden and towing a trailer, trying to overtake a tractor. Doesn't bear thinking about, does it? Post 1988 Mark 3 cars use the turbo diesel, and you might just find a cheap one if you are lucky.
Other diesel suspects: Talbot Horizon now very old (production finished in 1985) but worth a look if cheap enough, purely on the basis of its engine (the ubiquitous Peugeot 1.9 diesel). Goes well, roomy, but rust prone and not terribly well built. Citroen Visa has plenty of French eccentricity and 1.7 motor, still a few about. Daihatsu's 3 cylinder Charade supermini is reliable but out of its depth on the open road. Fiat Regata saloon boring, rusty, but even the best one in the world will be very cheap. If you need a big diesel, you might find a Ford Granada that hasn't been minicabbed to death, or a Peugeot 505 without holes in the sills. The Japanese were very slow to catch on to the diesel craze, but there are a few mid-size Toyotas and Nissans out there which will give good service until their bodywork crumbles away - beware parts availability problems though. Vauxhall managed to sell a few Carltons with the dated ex-Opel 2.3 diesel - good for watching the world pass slowly by in comfort.
Or for the ultimate in practical diesels, why not buy a Transit van? I'm not joking - they drive well (surprisingly car-like), you get the same brilliant high-up driving position as in expensive 4x4s and MPVs, parts are cheap, and you can hire it out to your mates at weekends and make some money out of it. You can drive it to parties and sleep in it afterwards. And the 2.5Di engine is a strong reliable motor that pulls like a train. What more could you want?