Anything I should know before buying a Micro Seiki RX-5000

lewm are the motors serviceable? 

To the degree that any motor is serviceable I would suppose the MS motors are serviceable. I know nothing about the specifics, but I do also know there are good new commercially available motors that might be suitable.

If you're buying ones with vacuum hold down feature, make sure the platter seals are in good shape. New ones can be bought (TechDas uses identical seals), but are pricey. Vacuum pump  and as lewm mentioned, motor health. AFAIK RX-1500 uses FG servo controlled DC motor.

Sorry no experience with the 5000.

I was only interested in non-vacuum hold down for fewer things to potentially go wrong

Firstly, be thankful you have the opportunity to buy one. These were lifelong purchases and do not appear often. Have the power supply recapped if required (mine is still fine from 1980) and install on an isolation platform, setup a few different arms and enjoy.