A/C in W123
I have a 84 300D (225K) and have had a 83 240D (200+K) and a 84 300TD (200+K). All of them, even when new, had relatively weak A/C performance around town. Until you were able to get going, without stopping, the A/C would blow tepid air. Even if you were only able to drive 30 mph constantly, w/o stopping, the A/C would work much better.
I know that with the conversion, the refrigeration system dosn't work as efficently, and I do not believe it cools as much. It may just be the "nature of the beast".
I assume you checked to make sure you have enough refrigerant in the system, as that could cause the symptoms also.
Good Luck.