The 603 idle speed can be adjusted mechanically to loose the computer. The idle control was just a driver comfort feature. Losing the engine computer also means losing the electronic transmission since they rely on a CAN-bus to communicate and work together.
The down side to using the 603 pump is you loose some power. The 603 pump van be turned all the way up to reach the stock 175hp level but it won't have the capacity to make more.

Another problem with the 606 is it requires an intercooler. The W123 doesn't have a place to mount the factory cooler so something will have to be adapted.

Good luck and post pictures!