Anyone know of an emulator that would run the old 68k version of NextStep?
You'd probably have to write one. I'd suggest paravirtualization would be the way to go, if you were going to run it on real 68K amiga hardware because as sure as MacOS needs the lower 4K, NeXT will probably want some absolute address range mapped that AmigaOS wants.
However, beyond any guest patches, the 68010+ meets the Popek & Goldberg requirements for hardware virtualization, so you can get away with ShapeShifter style "emulation", or virtualization as it's called in current parlance
