fiend: well amiga 1200 is the cheapest new'r amiga, and the latest of em all, only amiga4000 TOWER version came later.
amiga4000 desktop has alot of issues with older games and such (timer problems etc) , it does the job fine but i would recomend an a1200 to all, thats what i use and i am very happy with it (used it since 1992).
amigaone is the new'st amiga , and its ppc only , so old stuff that bangs the hw wont work unless u emulate it via uae etc.
amiga4000 tower (1995) is ofcourse the best choice of all, but its very expensive and costs heaps to upgrade.
amiga 3000 is from 1990/1
amiga 2000 is way older.
hope that helps