If you're just interested in making Paula-generated music (i.e. tracker stuff) then choice of Amiga isn't really important - every Amiga used the same Paula chip. You won't be caring about graphics or even a really fast processor for this type of music software. So an Amiga 500, 600, or 2000 would be a lot cheaper and do fine for running Protracker or any of the late 1980s, early 1990s trackers.
If you DO buy a 500 or 2000 you will want to make sure it has the ECS (Enhanced Chip Set) upgrade - that will give you the ability to have 1MB of CHIP RAM which is really important to tracking programs when storing sound samples. An AGA Amiga such as the A1200 will have 2MB chip RAM, which is nice. You can expand the chip RAM of an A500 or A2000 to 2MB with a DKB MegaChip (which is a little hard to find, admittedly), but you can still get a lot done with 1MB of chip RAM.
One big recommendation AGAINST getting an A1200 for tracker music (over an A500 or 2000) is that the A1200 has capacitor dry-out problems which can cause the audio to crackle or die out until these are replaced.
The A500 and A2000 are much better in this regard and don't often suffer from capacitor dry-out. They eventually will, but the A1200/A4000, despite being later computers, were made with capacitors that dry out much sooner than the earlier machines.
I'm not 100% sure, but it might also be that the A500/2000 designs use fewer capacitors as well. Certainly the circuit boards of the earlier machines make it easier to repair or replace components than do the later, multi-layer, more densley packed A1200/4000.