I’m sure DanC will add to this but here are things from my end:
1) Between 1 and 2% of the W113’s had the ZF fitted as an original option (SA230 and 232). Value as a loose item, between 5 and 10 000$, depending on shape and if all additional items are present (linkage, propeller shaft etc). If ZF box is mounted in an original car today and has original ZF serial No as per vehicle data card fitted, it will add even more than 10k to “normal” W113 value.
2) are you kidding? ZF is a high-end manufacturer
3) Maintenance none, except careful gear changing before box has become warm. Of course, rigorous oil changes. Spare parts: almost non-existant. We discussed here on the list to form a ZF group supporting each other.
/Hans in Stockholm