I don't want to discourage someone from having the true Amiga experience if that's what they are after.. I, myself, went out to buy several old machines that were already emulated very well, once upon a time. I had to find ways to transfer data to them (usually null-modem) which was cool, as I've always like checking out term programs, etc. The experience of what these machines were like was also very entertaining. But I was after that experience, not just making music. If you are looking to use real Amiga sound hardware (digitizer, MIDI, something like that) disregard the rest of this post.
With that in mind, you might want to consider buying a ROM from Cloanto and using WinUAE. I think it would make things much easier, not to mention cheaper. It wouldn't be like musicians did at the time, dealing with more restraints, etc. but you could jump right in a whole lot easier.
If WinUAE weren't as good as it is, I might not have this suggestion because of incompatibility. But, I think everyone here would agree that WinUAE is absolutely fantastic, and you wouldn't have any trouble making mods with it.