Our shop has probably worked on 3 300Es a day for ten years. I have never replaced an ignition control unit on one. I say this for two reasons. First I worry about the diagnosis and second it means that this is a very good choice as a used unit. I would be surprised if it couldn't easily be bought for less than 200. The reliability is so great I would not worry about a used one.
Control units in general are usually very cheap in the right junk yards. I bought a EA/CC/ISC controller for a 400E last month for $100. Turned out the problem was the throttle assy. I sure was glad that my "tool" only cost $100 not the $780 that a new one would have.
These controllers are so good that the junk yards wind up with piles of them. If you go to the extreme specialists here you will be stuck hard. Porsche yards usually get 50% of list price for such an item. If the car is common enough to make it to the large Import yards the prices will probably be less than 25% of new.
This advise is based on near thirty years dealing with Bosch electronics. It would be very different if the control unit was for an S-10 pickup truck.
Bosch Master, ASE Master, L1
33 years MB technician