I would not go the SSD route, as there are multiple cheaper/ more conventional alternatives, and the machine is not really able to take advantage of SSD speeds on the SCSI bus anyway. If I really wanted speed, I'd invest in a pile of 16MB RAM chips and set up a ram disk with a System and main software installation, using some of the old utilities which are able to manage it and preserve its contents on shut-down. But a RAM disk would be useful only if running System 7/8 rather than A/UX.
I have some suitable 50-pin SCSI hard drives here, or dial up Uniserver and ask him about the (newer, larger and faster) 2.5" drives that he has adapted for old Mac use (and ask too about their suitability under A/UX). I think Other World still sells the RAM.