I'm not old enough to actually use Amiga in the past. My 1st PC was... IBM PC something running MS DOS.
But definitely want to learn system programming on retro machines (beside GBA, Nintendo, etc. Just installed FS-UAE and FS-UAE launcher on my Mac. It asked you to provide the ROM of various Amiga machines (A500, A500+, A600, A1000, A1200 etc). I assume pick A500?
A500 has broadest compatibility of those listed. A1200 is the most capable of those listed (A1200 and A4000 had the upgraded "AGA" graphics chipset) but for emulation it doesn't matter so much as the emulators tends to offer expansions (like e.g. emulated "Picasso" graphics cards for something closer to modern resolutions and colours).
But definitely want to learn system programming on retro machines (beside GBA, Nintendo, etc. Just installed FS-UAE and FS-UAE launcher on my Mac. It asked you to provide the ROM of various Amiga machines (A500, A500+, A600, A1000, A1200 etc). I assume pick A500?