I have an 84 300SD w/ 240k with a strong motor but I think it needs a transmission rebuild pretty soon.

I want to tackle rebuilding one of these engines at some point, the transmission maybe as well. but I work alot of hours and this is my daily driver, so this time I gotta have someone else fix it quickly so I can keep on going to work. I have a local mechanic that I trust that isn't too expensive. he quoted $1500 to rebuild the trans, 400 if it is out of the car and I re-install it.

Or... I can get a well maintained 85 engine+trans from a crashed 300sd with 100k less miles than mine for only a couple of hundred dollars. can either just swap in that transmission (would that work with the 84 motor?)
or pay to have both the engine and trans installed in my car so I can keep driving it for a couple of years (quoted $1500ish), then rebuild my original myself at my leisure. maybe even soup it up with an intercooler, modified injection pump etc.

is this stupid to swap the whole engine and trans when my current engine is fine? too much work?

also, what exactly are the differences from 84 to 85? I know the tachometer is different, was there something different about the transmission too?

Mike