The first criteria, in my opinion, is whoever you select to assist you in pre purchase evaluation needs to be very experienced in sailplanes. Simply finding another A&P/IA does not assure you of this. It is possible that your best resource may not be an A&P.
In providing a pre purchase evaluation, I look at the following as a minimum:
1) Is there a complete history of the glider? Records are important and commonly not as complete as we would like.
2) Who did the last inspection? I expect this to be very recent. Does this inspector know sailplanes, especially composite sailplanes?
3) Evaluate damage history. This is not a deal breaker. Properly accomplished and documented, the only affect is any resulting weight changes. Note that damage history commonly does reduce future sell price as some people expect to buy only a glider with no significant damage history. This is somewhat unrealistic in older gliders.
4) Make sure AD's, service bulletins, tech notes, and periodic component replacements and weight and balance updates have been accomplished. More than half of the gliders I have done pre purchase inspections on have the following common issues:
a) Tow release out of service life.
b) Seat belt/ shoulder harnesses out of service life.
c) No recent weight and balance. Older than 5 years is a warning.
5) Physical inspection of the glider. This should be done to a written check list. Tap tests where appropriate. Evaluation of surface finish condition. Look at hinges. Almost none will show signs of recent lubrication of hard to get at control surface hinges. Control play check will identify wear. Look for signs of undocumented minor repairs, especially on control surfaces. Canopy condition is important. A properly done minor repair is no big deal. A canopy replacement is about a $6-8000 job, depending upon who does it.
6) Understand the storage history. Ships that live outside, not under cover, will have greater aging.
7) Evaluate the trailer. A trailer not well kept is usually a sign of a neglected glider.
8) Expect a written evaluation of findings.
This could go on, but hopefully you get the idea.
Aland Adams wrote an excellent article for Soaring magazine on this topic years ago.
Good luck with whatever ship you buy.
UH