If you wanna buy FL, you should at least take interest in the "FL Studio Fruity Edition", because the cheapest edition they offer lacks in "Piano Roll" and "Playlist" and believe me - these are a must. It's around 63 pounds - is that much for you? If I were to buy it, it's probably the cheapest music software with such possibilities ever to me.
I've got few mates who are beatmakers. They say that without a keyboard (I mean the instrument) it's harder to compose any music. So you might want to also buy one - I believe you don't need anything fancy; a basic one will do well. Also, they said it's nice to look for so-called "soundfonts" - these are banks of different instruments. For all I know, they are available for free. I've got 8gb of soundfonts.