For a casual hobby-type computer, I reckon the ultra-hackable A1200 is the business. Buy it cheap as you like, and start building onto it... It'll be expensive to add some stuff, and sometimes tricky to get working and reliable, but that's all part of the fun. If that doesn't sound like your cuppa tea, maybe an A3000 or A4000, which are also quite expandable, but for the more serious user/budget. It all depends on what you want use it for I suppose...
