A long time ago, when I started the upgrade/modifie trail on my P5, the MkI PSU was an early purchase. I did nothing for the TT, in fact it added a background hum through the speakers that the dealer nor Rega could fully eliminate, money back, no explination offered.. My much 'self designed' modified P5 performs superbly well without the PSU.
Go as sugested, via the decent cartridge route, when I say decent cartridge you are looking at a minimum of £500 IMHO . . . Then set it up properly, VTA shims and all, purpose designed digital scales and a lot of time evaluating the presentation. Alternativly enjoy what you have?
There is a third option in an aftermarket sub platter and a white belt, good value upgrade at around £200-£300?
Have you tried 'just a white belt', around £20.00, its worth it. In fact thats what I did those years back, the dealers sugested a white belt instead of the failed PSU.
CJSF