78-86 watch for broken exhaust headstuds, and look for upgraded chain tensioners.
86-89 check for broken headstuds particularly exhaust studs again, and there is some premature valve guide wear.
964, I have no idea on earlier models I think there was a cylinder to head leak problem, but I could be wrong.
I would highly suggest getting both of Waynes books, 101 projects for your 911, and his 911 engine rebuild book. The other book that would be helpful in looking into problem issues with model years is Bruce Andersons book Porsche 911 Performance Handbook. Before buying a car make sure that you have a Pre-Purchase Inspection (ppi) done by a qualified Porsche Mechanic. The books and PPI will save you 10 times the cost of finding out after the purchase.
You can get both of Wayne Dempsey's books here:
http://www.101projects.com/Porsche-101/index.htm
and I am sure pelican carries Bruce Andersons book
Rich