[KarmaKiller]

I think you got the right idea be building your own.
First thing's first, do you know how to put a system together?
If not, read this over, and then see what you think. It's really rather easy. Just make sure you get compatible parts, and just be careful and use common sense, and you'll be fine..
I have a question on your budget though. You first list £1200GBP as your budget, then later in your post you say your trying to keep it around 500. I'm assuming thats your money?
Post back that, and I'll throw together a list for ya!