Mileage should not affect the reliability of the car, provided that it has been maintained regularly and correctly.
check out the usual 996/997 stuff:
check if the suspension and in particular, the lower control "coffin" arms / shocks have been replaced. Check the discs and in particular the rears as these can corrode badly on inside face.
There has been loads written about engines here, the usual suspects are IMS, rear main seal and smoke from one exhaust which can indicate ring or bore wear. This can run to 8 grand if it is in need of new set of liners.
if it is a manual, they can pop out of second which can be a synchro Problem and around 2 grand to fix and the C4 front diff can weep oil.
Ask if the rads have been replaced as well, or if grilles have been fitted to stop leaves and other damp crud getting lodged in the rad area. New rads are a few hundred to replace.
Otherwise, it is just checking for accident damage (check for front end damage by lifting the front boot space carpets and looking for ripples in the panels)and general wear and tear due to usage which is to be expected on a higher mileage car.
Make a list of any faults you come across and just be prepared to spend some money, but buy at a correspondingly lower price.
Lastly, you can get an independent inspection made if you like the general condition of the car and am sure Richard H will point you in the direction of FAQs on the forum for more detail.
