Sparcstation 4, 5, 10 and 20 models (and the Voyager) can run NeXTstep. For the 10 and 20, it depends on the CPU. Sun SuperSPARC modules should work, but Ross HyperSPARC CPUs are not supported. NeXTstep only supports one CPU, additional CPUs are ignored. Later models (UltraSPARC) are not supported, unfortunately there is also no support for earlier SuperSparc Sun 4m (e.g. the LX) or Sun 4c systems.
I have a SparcStation 20 with 512 MB RAM and the 24 bit framebuffer RAM expansion, this is a great machine for NeXTstep.
If you happen to have an hp PA-RISC machine, some models using the PA7100 CPU (e.g. the "Gecko" 712-series) are also supported and are really nice NeXTstep machines.
Vintage Apple hardware does not work with NeXTstep directly, but you can run Rhapsody (essentially NeXTstep for PowerPC with a System 9-like UI) on early G3 Macs.
The Motorola 88k-based NRW (NeXT RISC workstation) was unfortunately canceled when NeXT decided to leave the hardware business...