A couple of starters for you.... these are just a few of the things I looked out for when searching for mine... A couple of them (fuel guage & ignition switch) I only discovered after purchase though!

Creaking suspension - coffin arm bushes are shot, front or back, lots of bushes, the tuning fork links fail as well, together with top mounts.
Damp/ wet passenger side rear carpet - roof drains blocked. Central locking Ecu is under the passenger seat and the water eats into it.... Costly to replace!
Fuel guage - the sender/ pump is an integral unit in the tank and the sender can fail.
Ignition switch can fail - doesn't spring back to Pos II after starting
IMS and RMS although I believe it's less so on the 2.5 than the 2.7 or 3.2. I understand clutch judder is an indication of oil leaking from the IMS onto the clutch plate/ flywheel... Less common on the 2.5, be aware it is a DMF, a clutch will very often need a new flywheel.
Front radiators can rot - leaves collect in the vents at the front and sit against the bottom of the rads rotting them out. Check for debris., also knackers air condensers which are in front
Springs can snap causing the car to sit lob sided
Windows should drop slightly slightly when opening the doors - open them slowly! They also stick/freeze in winter, handles break.
Exhaust clamps/ hangers corrode and break Together with heat shields.
A little puff of smoke is normal at cold start up any more is a potential problem

These are perhaps the Boxster specific items other than the usual checkpoints for a used car search... Hope this helps

Good luck