I have retrofitted a W123 300CD a few years back, as well as Ford products without problems or destruction. On the W123, replaced the Delco compressor with new, new drier, new expansion valve, new pressure switch, and new o-rings. Used the oil compatible with the R12 oil, filled it up to 80% to 85% of the R12 fill and it worked fine. Note: System was sealed and compressor had not self-destructed, so didn't purge but just blew out the lines with dry air. The center vent temperature was below 40 degrees F (38 if I remember right) in roughly 85 degree weather. Just drive the car occasionaly, but have used the A/C and it still works fine. Do have a problem leaking out refrigerant though. I think those adapter fittings for the fill connections are crap, and may cause a leak depending on which kind you get.

A working aux fan is a must, and a larger condensor I'm sure can't hurt. If you put guages on the system, and watch when the fan kicks in, The high side pressure really drops a lot. Getting rid of that heat is important for efficiency. Now that I think about it, I wonder if making the fan go on all the time when the A/C kicks in rather than based on high side temperature, or adding a larger fan, would improve the cold air output.

I've had very good luck with conversions, other than having a hard time keeping the systems full. As you said, if it is done correctly, R-134a should work about as well as R-12.

A note on the hoses. If you replace them, make sure they are rated for R-134a. If not, I doubt they will stop any leaks. I read an article or two where experts say that older hoses actually reduce leaks since any pores have been filled with oil and plugged over time. Don't know if there is any truth to that.

P.S. Others have stated that the R-134a systems have higher pressures. My experience has seen lower pressures on both high and low sides than with R-12. I don't know if it's due to the specific systems I have retrofitted, or if this is a general rule. Maybe others with higher pressures are using too much refrigerant.