tranny rebuild Question

I have decided that the time has come to rebuild the transmission in my 450SL. It has been slipping and skipping 2nd and clunking on the 2-1 downshift. After searching many shops in my area I found a rebuilder that will do the rebuild fro $650 with torque converter. This price seems(strange to say)too low. What questions should I ask him about rebuild parts used etc. before I consider him?

A little nervous
jmdunn

77 450Sl
Med. Blue/Parchment
173,000 miles