The problem with 32 colors is that you can't move so much objects on screen (well, less than when in 16 colors mode) and for a shoot'em up it's quite cool having many sprites to shoot ! ;-) In lowres 16 colors, the Amiga blitter can move +64 bobs (16x16 pixels, 8 colors per bob)
This is for unexpanded machines though? And most Amigans still around either have expansions or use winuae.
I'd put the baseline at 4 possibly 8mb and 28mhz processor.
It would be nice to see what that spec could achieve, it didn't get many pro games, as we all had that towards the end of the Amiga's life.
What little time I have, i'm willing to donate. Iv'e been a video games artist (mainly 3D and texture maps, but could certainly turn my hand to paletted tiles). for the last 13 years, with some design credits also.
I'd certainly want to make games with a decent engine though, although my main love would be to do a dungeon master style Sci-fi game with rendered graphics.