I think your problem lies in the McParts starter. Double check the water level in your battery, voltages sound OK, but check the level. Haven't heard much good about anything less than a Bosch or MB reman starter.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.