Another option is some games such as Left 4 Dead 2 have all of their sound folders open and can be replaced with any other .wav file (providing it's the same length), theoretically allowing you to rescore the entire game. It's great because you can compose a piece, and then experience it in game as an interactive piece of music.
Following that you can release your score to the Steam Workshop as a mod for people to download.
If anyone knows any other games with replaceable sound files I'd love to know!
Thanks Ravyne, that's actually a really good point, there's not much use paying out for a PC now if I plan to replace it soon.
I've actually done some looking into Mac Mini's - and by the time I've bought a monitor and expanded the RAM it comes pretty close in price to an iMac, and not as highly specced. I think prices may be steeper here in the UK
I think I may just hold out and buy an iMac (possibly refurbished)