newbie with ?'s on 90 300ce
 

looking at a 90 300ce,built 12/89 with the m104 24v it has 156,000 miles,i have been reading up on these and they seem to have a few issues,my questions are 1.is the 90 affected by the engine wiring harness problem,i see lots of mention of the 91 up but nothing about the 88,89,90,and if so what does a harness cost?? 2.they seem to have water and oil leakage issues,first if it is the timing chain cover leaking water is there a particular place they tend to leak,3. i here they leak the oil on the right rear corner of the head,this car has oil residue on the egr pipe up by the valve cover gasket ,and it does look like the cover gasket is leaking or i should say seeping very little the side of the block down low is damp but not wet.4. what kinda milage can you get out a head gasket,i have no history of the car right off hand .