A manual system would be my first choice, but parts I would have to buy! My 1983 300D's and TD's with the newer automatic system have not given me any problems in years with the exception of a Programma that decided to spit out it's parts onto the passenger side seat at 116*F!
But that was easily fixable in one afternoon and this system is one I already own.
That has a lot of influence too.
The 1980 300TD is also a wagon, which will fetch a decent amount of dough for an Arizona Rust free chassis car even if it needs the new climate control system. The front seats are out and the center console is in the back, so it's mostly taken apart already.