I'd have to say that you aren't going to have much of a shot at building a cab for 200 pounds, unless you already have your wood and tools or you can find an empty cab really cheap.
It sounds like you need a computer as well which won't be cheap.
I'd say maybe try building yourself a control panel first and playing that way for awhile until you get some more cash.
As far as computer power, parts, etc., please read the FAQ and look at the stickies in the forums, these questions have all been asked way too many times.
Good luck!