I don't see why a GUI should not work. Perhaps even using parts of the Mac ROM, if they do not conflict with Minix's paradigm.
Minix doesn't require an FPU, but an MMU is necessary for memory protection and virtualisation capabilities. Of the old OSes that used the capabilities of the 68030, only A/UX did (although earlier 68030-based Macs didn't use the burst mode of the CPU due to economy reasons.)
Also RAM should not be much of a problem; and the upper bound would depend on the hardware, not the OS. I reckon a system with at least 1Mb should be able to run a basic Minix system. If other 68k implementations are an indication, even 512Kb should be sufficient.
Minix doesn't require an FPU, but an MMU is necessary for memory protection and virtualisation capabilities. Of the old OSes that used the capabilities of the 68030, only A/UX did (although earlier 68030-based Macs didn't use the burst mode of the CPU due to economy reasons.)
Also RAM should not be much of a problem; and the upper bound would depend on the hardware, not the OS. I reckon a system with at least 1Mb should be able to run a basic Minix system. If other 68k implementations are an indication, even 512Kb should be sufficient.