OK, I just have to ask - why stop at 4MB RAM? Why not 8MB, or even 10MB?
My old A500 has a GVP HD8+ and 8MB FastRAM - on Minimig, with ARM controller, lots of floppy drives and HD images, the more RAM the better. 1.5MB chip + 8MB "other RAM" would be excellent for a machine like this, and much more fitting for the 28MHz CPU. Anything less, and there wont be much benefit over Minimig 1.1 + ARM controller.
Oh yeah - RTC, dont forget battery backed RTC.
Heck, while I'm on it... add cheap ethernet controller, maybe with a RTL chip, or simulate lance/amd am7990 (A2065/Ariadne) in FPGA or something, and it's best amiga clone ever already
