There should be a label with a date on it where the wiring harness goes into the engine computer. In my E420, it's right near the passenger side firewall. Not sure if there's a battery there on the 300E.

As far as the evaporator - you may be able to read a date code stamped somewhere on the receiver / dryer. Not sure though. This part should be behind the driver's side headlight (it will have a glass sight facing straight up).

Head gasket? Not sure.

Hoep this helps.

Troy
1995 E420 80k