The fact that you couldn't find any posts regarding these starters gives you a pretty good idea how reliably the typically are. The original starter is in my 300E after 267000 miles and has never been touched.

When the starter is failing to work, you need to measure the voltage at the starter, not the battery, while you have someone turn the key. If there is voltage at the solenoid, then you need to check the solenoid and starter, if there isn't, you need to go backwards through the switch and see where the voltage is not making it.

You really need a wiring diagram. The one in a Haynes manual should do.

Good luck,