I'm taking my 85 300sd in this weekend for repairs on the a/c, we are planning to wire the aux. fan through a fuse/relay directly to a independent switch, so that I can switch on the fan when I put on the a/c. Is there any foreseeble problems with this setup.

There is absolutely no need to do this. The aux fan is there to bring down the high side pressure when it becomes too high. By turning on the aux fan, the added flow across the condensor reduces the pressure.

Just jump the high side switch to ensure that the fan is working and go with it.

Good luck,