You can start out with the one you like the most, then when you get hooked you can start collecting other models. One thing to keep in mind is if you want to transfer ADFs to it you will need some extra RAM and a easy way to transfer them from PC to Amiga. So maybe a Accelerator with Fast RAM and SCSI would be cool that way you can transfer files on a PC Zip disk or something. A HD would also help out too so you can work easier on the Amiga. Also if you like games you will definately want to use WHDLoad. There are a ton of games you can run from WB, but again you need OS 2+, Fast RAM and a HD.
I myself have a A4000D and A3000T on my home LAN, so I can transfer or download ADFs to the Amigas to transfer to real disks and play most games with WHDLoad...
