Hello, I have an 1980 w126 500sel that I having trouble to get started. At first I thought it was the battery because when we hook it up to jump it, it takes a while but finally will turn over and start (after turning the key numerous times with it sometimes turning over and clicking but sometimes not), but as soon as you stop the engine and try to turn the key, it will click once and not turn over. Tested the battery it is at right under 12V. Any other suggestions on what I can check? Much appreciated....

Sounds like a bad starter solenoid. When it just clicks, put the tranmission in gear and push the car back and forth a little to get the starter commutator to move. See if it will make contact now and start. If so your solenoid is going bad and probably has a flat place on the commutator (at least that's what I remember it being called). Been a long time since I have messed with one.

as mentioned previously it may be the starter solenoid or the transmission safety switch . Try to start alternately in N and P. if it clicks in both positions best to replace the old starter unit .
regards
A.