BW,

I've swapped a number of manual 240D transmissions into Turbo Diesel 123 Series Mercedes and we've never had a problem with the trannys holding up. Like you I've considered staying with the 617 engines and putting them in everything else I'm interested in and I'd go the 240D tranny in an instant.

I've five 123s and all have been converted to manuals. Everything from 240D, 300D and 300DT. Go for it.
Ben