It all depends what you want to spend & what it's used for
A500 - Nostalgic games, cheap memory, expensive accelerator = Expansion viable at a cost (hardware rareer).
A600 - Nostalgic but does not run all A500 games, additional IDE HD or CF card, can be used on the internet with PCIMCA network card, 030 accelerator card (Amiga kit) & V3.1 Rom
A1200 - All the same as above but with AGA (Advanced graphics) & more availiability of expansions from a wider source of areas.
A1000. A2000, A2500, A3000 - all availiable for expansion but may cost more money as hardware is not so as easily obtainable as the previous Amigas.
A4000T (Very Rare), A4000D (Rare-ish) these machines are fully expandable at cost but are a very good option if you are patient and have a disposable income with no limits.
I have all these Amiga's except the A4000T and the one I use most is the A600 with V3.1 ROM's as it's smaller and does not take up much space.
Good luck in your choice of machine, they are classics and are all very good at what they do with the various restrictions and hardware.
