You can manually force a 300SL and other models to start in 1st gear. With the car at rest, if you move the shift lever to "2" the transmission will shift from 2nd gear to 1st, and you can return the shift lever to "D" or any of the other forward positions and it will remain in 1st. Might be interesting to see what impact a 1st-gear starthas on the problem.

The 1990 has an idle speed control valve. Your RPMs are low with the A/C on, but if your car idles at higher RPM when cold, I would guess the valve is working. Maybe the compressor is binding internally...If you reach down and turn the clutch on the front of the compressor with your hand it should move with little effort (engine off, of course).

Regarding the cruise control, unfortunately, you probably need a new control module. If you have a voltmeter you can check the inputs and outputs to the controller to identify the problem. If you want to do this yourself, I suggest starting a new thread on the topic. I can give you some basics and others who know more than me can contribute.