A good recommended independent would be my choice. Word of mouth in the pcar business means a lot.....
I would recommend while asking for a PPI you also give them some focus items especially with car having such low miles (as you mentioned..seals, rms, boots etc). The cost depends on how indepth they go and how good they are...a good mechanic should be able to run through it pretty quickly but you don't want it just combed over. You should also be run through the PPI at the end so the mechanic can show you what was good / bad. I have seen between $150 and $200 for a PPI...anything higher and you should push a bit harder IMO. Note though that at $75/hr your looking at 2 - 2.5 hours...at $100/hr that will push the price up.
There is some good info on buying a pcar in these two links:
http://mike.focke.googlepages.com/buyingorsellinga1997-2004boxster
http://mike.focke.googlepages.com/buyingcontinued
The second link mentions things to look for and the information is very good.
Good luck with the car...and I hope it all works out....