Hey, congratulations on the car. First, probably have had limited responses to your note because much of what you have asked can be found by doing searches on this site. Second, machine gun blast of questions would take hours to respond to.
Please see below for some quick responses:
A new owner (2nd) of a prestine 1989 300E from an very mature lady. 54,000 original documented miles. I have some questions and problems. Where exactly is the engine #? Cd manual will provide you with that, also, believe it's stamped on motor down low on passenger's side above oil pan. When replacing the front drive shaft rubber ring do I also need the mounting hardware? What do I have to do to unlock the radio, it has 4 dashes accross the display? call dealership with vin and they will provide you with the code, also if you got the manual and owner's documents it should be in there ABS and SRS lights are on. sounds like based on the fact that you need to reset radio somone may have jumped the car and crossed the cables blew out OVP fuse, it's located behind the battery under a thin black plastic shield, two 10amp fuzes on top may have blown (did not come on when I test drove (10 miles) before purchase. Does engine require filter with check valve? What kind of gas mileage? [I] about 27 mpg highway [I/]
What are procedures for switching A/C to R134a? What RPM should you see at 55mph? Can you reset engine timing for less than premium gas? I have a Service manual on CD coming from Mercedes.