I've been testing out 2006-2007 Focus and Mondeo TDCis after working out that I would save £600-700 in fuel a year by driving a diesel. I don't do particularly big mileages either.
The 1.8 Ford TDCi engine is supposed to be more reliable than the 2.0 or 1.6 versions. The 2.0 and 1.6 are known for developing recurring injector problems which cost £80 quid minumum to put right at an authorised Ford service centre and independent garages are mostly incapable of sorting out. They also don't like short journeys and Diesel Particulate Filters (DPFs) are expensive when they pack up.
Have tested Mondeo 1.8 TDCi hatchback, Focus 1.8 TDCi, and Focus 1.6 TDCi (both focus estates). The 1.8s had rather unpleasant turbo lag issues and the 1.6 was worse. The 1.6 was particularly poor on hills until the turbo kicked in. I know that if I were to buy one of either I would quickly develop a very heavy footed driving style - or possibly go the other way and drive like a nun to avoid the turbo. Either way would either be not fuel efficient or not much fun. I drove a W Reg Skoda Octavia 1.9 TDI and it put both Ford engines to shame. A much more smooth, enjoyable drive and had a similar amount of poke when I did put my foot down. Noisier when starting and idling though.
Clutches on the Ford Mondeos and Focuses tend to pack up somewhere between 80k to 100k if not before. A lot of adverts make a big thing of it having been done. I think it's about £300 ish for a new one.
If they're cambelt driven rather than chain driven, make sure the belt has been replaced at the right intervals.
Mazda 6 in your price range (few and far between I know) uses the same 2.0l TDCi engine as the Ford. The newer 2.2l Mazda engine in more recent Mazda 6s is supposed to be a more reliable Mazda designed engine but I've only ever seen one listed for less then £10k so far so not much good without a further couple of years worth of depreciation.
As for whether to get a petrol or diesel car? Well there's economics but some diesel engines are better than others. Obviously if the equivalent petrol car is cheap enough to offset fuel savings, it doesn't make much sense to get a diesel.