The 92 500SEL probably still has CIS "mechanical" fuel injection with electronic control. This is big $$ to repair and used parts don't age well ( RE fuel distributor )

The 94 S420 should have full electronic. This is one of the years that suffers from "bio degradable insulation" engine wiring harness failure. The harness is easy to change and some updated used ones are floating around the classified on this list so don't let that put you off.

Do some research on timing chain guides and oil tubes for the cylinder head / lifters. From what I recall these were common repairs. ( Any car regardless of brand will have weak points. )

Past that I don't have any other detailed info.