I just bought a 1990 C4 (although I still don't have it yet  ). It's my first so I'm no expert myself, but did a lot of research and have a couple of threads that might help you.
Let's Talk About High Mileage C4s (again) - Potential Purchase - Rennlist Discussion Forums
What service would you give a car with spotty service history? - Rennlist Discussion Forums
Here's a good overall info website: John's 964 Web
The Rennlist 964 board is very active and has a lot of good info over there. Not that I'm trying to steer you away from the Pelican boards! 
Just a start anyway. Good luck with your search!
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Doug
Currently Between Porsches
PART OF MY SOUL: '09 Boxster 2.9 PDK, '86 911, '76 912E, '06 Cayman S, '90 911 C4, '74 911, '78 911 Targa, '01 Boxster, '70 911T, '99 Boxster (#2), '72 911T, '88 911, '99 Boxster (#1), '84 911 Turbo Look, '73 911 Targa, '88 944
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