While you are going about this in the correct manner by really analizing the problem at hand, if you have it narrowed down to the starter, I would just replace the starter. It comes with a new solenoid and a warranty.
The reason for this is the labor is not cheap! You don't want to take out the starter two or three times to fix one problem.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS