Paul,

Don't go crazy here. I've converted a 123, 124, and a 126 to 134a and have achieved good results each time. The 124 was a 1986 300E. When the evaporator started to leak (it seems they all will) I replaced it, removed as much of the oil as I could, evacuated, and charged with 134a. It was not quite as cold as the R12 system but it worked fine. Keep in mind I live in Florida where the humidity and heat load from the sun is very high.

I eventually removed the pin from the visco fan clutch which locked it up at speeds under 3000 RPM. This car was used almost exclusively in town and the extra airflow improved the system at idle.

If your compressor is good, leave it. The hoses will be fine. They are impregnated with oil which will be a barrier for the 134a.

So:
1. The parallel flow condenser. If you feel you must but I wouldn't.

2. No reason to bother. I ended up topping off my cars of every 2 or 3 years.

3. Just make sure the fans work properly.

4. If the compressor is good, use it.

5. New receiver dryer, new expansion valve, suck down the system for at least 1 hour, and I think you will be happy with the results.

Any residual original oil will find a low spot and won't be a problem. Don't over oil the system with PAG oil, use the amount recommended by MB. Charge with 134a to about 80% of R12 charge. It seems a little under is better than over.

This is not a big deal, conversion is straight forward. My 89 300SEL is pretty darn cold. My wife is happy and she hates the heat!

Good Luck,

Tom
92 500E pearl gray
92 500E black pearl
89 300SEL needs paint