If you're interested in running A/UX today, assuming you can obtain a copy, the ideal hardware would be a Macintosh Quadra 950. These machines have a 33MHz 68040 processor, good video, and the capability to use lots of RAM. Its dual, or triple with the PDS expansion card, SCSI busses also allow a fairly respectable amount of mass storage to be attached.
I have one of these at home running the most recent revision, and it makes a surprisingly capable workstation. Most old Mac OS apps will run on it, and though the CPU is fairly glacial by today's standards, it runs very well. Because A/UX includes an X server, any applications that don't run natively can be displayed remotely, while running on a newer machine. For example, I use Firefox running under AIX on my RS/6000 for web browsing, and thanks to NFS, my downloads can go directly to my home directory on the Quadra.