my 930 has just over 160,000 miles on it and is now due for a top end rebuild only. just for ease of mind i'm taking care of bottom too even though my mechanic said it doesnt need it. If i was you i wouldnt worry till you have to take care of the top; which will usually go first...
1979 Porsche 930: 3.4L, SC cams, Twin plug, Leask WUR, Custom SSI turbo exhaust, Tial WG, K27HFS, and we can't forget the Zork (short lived depending on my homeowners assoc.)
05 Boxster S: For the Track.
06 Dodge Ram 2500 Power Wagon: Tow Vehicle