@ArmouredWizard
1: Diesel. You're looking for a high mileage car on that budget, and diesels will stay more reliable for longer.

2: Smallish engine, 1.4 or 1.6. Cheaper to run, plus it's unlikely to have been owned by someone who liked to thrash it about.

3: Independent local dealer, if they offer a warranty.

4: Get underneath and look for corrosion. Get the cleanest one you can find, particularly suspension.