90 to 93 300SL do not have engine wiring harness problems. They were CIS engine management rather than the HFM in 1994. The 104 head gasket problem is not terribly predictable. I've heard of the problem as early as 20k miles. I had a car that needed new head gaskets at 35k and 68k. Some cars don't have the problem at all. It's really tough to predict although 104s seem to be a little more prone to it than the 103s.