There are some great deals around for £3,000 or so which will get you something around 5-7 years old and with 30-60k miles on the clock. If your really clever you can get some with around 10-30k miles on the clock for around £3000.
Really aim for a car that has done 10k miles or less per year as you don't want to be sat on the side of the road waiting for the AA to tow you to a garage just so you can spend another few hundred quid on repairs and labour.
If you can get a car with 1 lady owner, you are probably going to be onto a winner in terms on reliability, especially so if they are elderly. With this in mind, you probably wont find a Mitsubishi Evo with 1 previous elderly lady driver.
If you live in London, I would suggest travel further up north where cars tend to be much cheaper. I recently saved around £800 by spending £100 on a train ticket to Chester. If do travel a substantial distance to buy a car, do your best to buy from a main dealer. I did this and my car came with a 12 month warrantee so now if I have any problems, I can go to my local Toyota garage and (hopefully under the warrantee) have the issue fixed.
Check out some blogs, there are some useful ones, to get some useful car example ideas.