Jardo,
The SP-15 doesn't have an outboard PSU - all circuitry is internal. If all the wires have been cut off (many were hardwired into their consoles) then you'd have to connect a standard AC power cord and plug.
That said, my main deck is an SP-15 in the Technics SH-15B3 plinth. I bought it in a Technics ProBase, which is just a thick piece of Masonite with a cutout for the motor, and the SH-15B3 is far superior. It improved even more when I replaced the stock feet with cones. The molded rubber of the SH-15B3 is very dense and non-resonant, and the replaceable armboard is ideal for trying different arms.
I've never tried any other plinth materials, although people build all sorts of different plinths for their SP-10s. The SP-15 might be improved with a similar treatment, but it might also be overkill. The SP-15 is a fine motor, but "right up there with the very best" might be stretching it. If the circuitry starts to go, the body of knowledge and availability of after market parts is much smaller than it is with the SP-10. On the other hand, you can buy an entire SP-15 motor for $200-300, so repair isn't really cost effective anyway.
Anyway, have fun. I'm really happy with my SP-15, and I'm about to mount up a Micro Seiki MA-505 which should make it even better.
David