Being an air cooled convert my opinion is totally biased, having said that I know there are a few common areas of concern for the original 911's; head studs, hydraulic chain tensioners, valve guides, etc. If it were me, a PPI with a leakdown test after much research into the series' general strong points and common maladies;
The Pre-Purchase Inspection by Peter Zimmermann - Rennlist Discussion Forums
911 Forum - Rennlist Discussion Forums