With Larry's help, and others on this forum, I repaired my AC earlier this summer. It was on a 1989 260e - so I can offer my experiences:
I did the r134 conversion and have been happy. But I live in Cleveland, OH - where the average high in the summer is around 80F and humidity is not that bad.
Here's my numbers for charging:
I put two (2) 12oz. cans of r134 into a fully evacuated system. Which is 24oz., which is a 1.5lbs.
I really wanted 2 lbs. (MB recomends 2.4lbs for r12 - and 80% of that for the r134 conversion is 1.95 lbs).
But I didn't want to over charge - and they don't sell 8oz cans of r134 at my local part shop. But it blows cold. Anyway, here are the numbers....
With an ambient temp on this particular Saturday of 85F:
At idle (~700rpm)
The vent temperature as the car was sitting in my driveway at idle, was about 55F.
As I drove east on I-90 at about 70mph, the vent temperature got down around 40F.
I found that a nice way to monitor the vent temperature as you charge the system is to use one of those indoor/outdoor digital thermometers with the outdoor probe on about 4feet of wire. I got one at Kohl's department store for $14. I get all the air flowing through the center vent - full cool - full fan - and put the outdoor probe into the vent opening. I take the digital part outside of the car through the window, close up the window - and watch the vent temp drop as the refrigerant fills the system. Mine actually has a woman's voice (sounding like she popped a fist-full of valium) saying "The outside temperature is XXX". Kinda' useful to hear if you aren't looking at the unit.
I drove around with the thermometer in the car and could monitor vent temperature as well as cabin temperature while cruising I-90.
Great tool for AC work. But when you're done, be sure to hang it up in your kitchen before your wife notices it was missing. Oh -and don't forget to wipe off the greasy finger prints.