I have a couple comments (I am the guy who converted his '84 300D to a parallel flow condensor and 134a):

1) Is there a particular reason you are going to replace the evaporator? You said it was just replaced a couple years ago. If it isn't leaking then all you need to do is flush it and you'll be fine. Replacing a perfectly good evaporator seems like a waste of time and money.

2) Definitely have the a/c shop install a filter.

3) I did the 100% recirc trick and didn't like it. Here's why. People say there is 80/20 recirc., and I found that not to be true. What happens on my car is the recirculation door opens and closes on some cylce (the system does it automatically). Sometimes the door is closed completely, giving you 100% recycled air, and sometimes it opens, giving you a mixture. I closed it completely for testing and DID NOT notice a significant change in a/c temperatures - maybe only a degree or so. Besides that, I really was unsatisfied with the fact that I had somehow butchered my car and undid something Mercedes had desinged it to do. If you repair the a/c system properly, you WILL have good cooling without jury-rigging the recirculation door. Lastly, its nice to have outside in the car, which you won't have if you alter the system.

4) Jury-rigging the electric fan may be a good idea, but try it first without doing so. If everything is working properly, it will turn on when it is needed, but you can test it later and see if your vent temps are lower. I prefer leaving the car stock when possible.

4) Like everyone says, just go with the r-12. The on-line test is not difficult, and if you are serious about doing the best job you can, you won't waste your time with 134a.

GregS
'84 300D, 174k
'90 300CE, 164k