100 gbp isn't going to get you very far, I'm afraid. On estimate, you're looking at around 300-400 to get it looking and running
really good. I suggest to start looking for a P5 1260 or BlizzardPPC. They're not so easy to find, but they do crop up every now and again. That'll set you back a cool 200 right there. The next biggie is PCI in the form of a Mediator. Software Hut and Amigakit both have them in stock. Thats another bill, at least..
Once you get those two essential items out of the way, then you'll naturally want more.

You can really go kinda nuts upgrading Amigas.. I mean you have Zorro slots, PCI slot, multiple clockports, even a cardslot! You could have three of everything if you really wanted to.. Its only really limited if you are married. Welcome to the wonderful (and expensive) world of classic Amiga.