I'll answer what I feel comfortable with, but as the previous poster said, all of this has been covered and can be dug out using the SEARCH facility here.
ABS/SRS light on - very likely the bruses in your voltage regulator are worn. Replacing these is VERY easy. Put front of car on ramps. Pull large plastic shroud underneath engine. On passenger side below you'll see the alternator. 2 phillips head screws on bottom of alt. hold in volt. reg. New brushes are about 1/2" long. Yours are likely going to be much shorter. You remove the 2 screws, pull out the unit, stick in the new one and reinstall the 2 screws. It's that easy.
Timing - Your car has a knock sensor that will automatically adjust timing. Many people here and on other MB boards have reported using mid-grade gas. Several pro mechanics have said that the 103 motor actually runs better on mid-grade vs. premium. I use premium in my 103.
RPM - Don't know about 55mph, but at 70, I see about 3400rpm. You may be wondering why your car only shifts twice before reaching top gear. They start out in 2nd by default. If you want to start out from a stand still in 1st gear, either stomp the accelerator pedal(this will kick it down), or while stopped, drop the gear selector down into the lowest gear; then back to D.
Forget R134A - stick with R-12 - just my opinion.
Last edited by Mike Richards; 09-29-2002 at 11:36 PM.