This may still be a little far for you depending on which direction the 100 miles is, but ...
Pico Blvd. at 10th Street
Talk to Rick. They are VERY, VERY thorough, recommended by Simonson, the dealer in S.M. The shop is about a half an hour, maybe 40 minnutes from Los Alamitos. The charge should be about $85. They saved me from purchasing 2 cars wherein the owners (private parties) had gone to great lengths to mask some serious but tough to detect problems. They are the best I have ever had look at a car for me. Worst case, if you can't take it to them, is they may refer you to someone more convenient.
The following I have NO EXPERIENCE with, but they were recommended to me. I almost bought a car from them.
Tilo's European ********
5300 Katella Ave.
Los Alamitos, Ca
By the way, they just happen to have a '91 560 SEC with 110K on it for sale. Could it be they have it on consignment from your PP? What are the odds of 2 560sec's with 110k being in the same city? Hmmmmm.
Any competent shop will cover all the bases and probably knows the idiosyncrasies and weak points of the specific model. Do ask them to check the body for replaced or repaired body panels.
Good luck. My wife still hates me for not getting an sec or ce instead of the 4 door.