System 3 isn't horrible, but it does take some work to rehab a worn out system. I think for about $350 to 400$ in parts you can refurbish the "control" side of the system (as opposed to the refridgerent loop side. The refridgerent circuit is pretty much the same on all w123's I think, so I am not going to go down that rabbit hole.

HVAC pods - one of the members here sells rebuild kits for these. I am blanking on his name. Roughly 40$ for all new diaphrams. Pulling the dash to rebuild them is not 100% necessary, but honestly it's not as bad as people make it out to be.

Switchover/changeover valves (i get them mixed up). Not incredibly failure prone. I had 2/5 failed, replaced those with ones from a junkyard. Can be accessed with the dash in place.

CCU - yours may be burnt out. Mine was, tried to mix & match parts from another unit with bad switches, not quite compatable. Bought a refurbished one.

Aux water pump - easy to replace, don't recall price. Add a 2a fuse, easy job.

Cabin temp sensor - can actually be rebuilt for about 5$. I think I have instructions posted, if not ask me and I will be glad to help.

Monovalve. Huge pain. Find an original core that works and rebuild it.

Personally, I prefer to have an original system that works. Not sure how difficult a conversion to manual controls is. Might save you a bit of money, but I doubt it's easier than rebuilding ACC3. Though to be faie I have some electronics skills that others likely do not have. And I am really picky.