I've replacd a lot of things on my 1992 240, and I was wondering what I'd do if the transmission went out. It's an auto, and I've always wanted a 5 speed. Or a 4 with electronic overdrive if a 5 won't work. Anyway, my question is, on these later model bricks, is a transmission swap possible for a reasonable price, and am I going to have to do a bunch of electronic crap to get it to work? Thanks!

I would go the M47 route, for simplicity sake. No wires (except the B/U lamp). The rear gears ratio is a non-issue. Yes the A/T cars have steepre gears than the factory M/T cars, but in any event you will not be turning more RPM than you do now because both the A/T and M/T are OD transmissions. Same to same.

You will need a different front driveshaft, or have yours adjusted for length.

The clutch is easy, remove pedal bolt, install M/T pedals, run cable (hole already there, but block-off cover in place), install new M/T flywheel, clutch, and transmission assembly (complete with release fork and rear mount) and you are done. You will likely need to move the rear trans crossmember forward or back to match up with the M/T, but the holes are already there.
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JohnG 1989 245 MT @216,500

An m47 5 speed would be the easiest because you would not need to instal the dashboard wiring harness and relay that an M46 overdrive would need. But even installing the harness is not a big deal, since there should be a plug sitting behind the instrument panel for the harness (the only complication is the the SRS light takes the spot of the OD light for an m46).
The problem with an m47 is finding a good used one. They are prone to bearing failure. A good M46 is probably a better bet, and they are easier to find. You should consider having the OD seals done on an m46 before installing it. For either tranny you would need the pedal assembly, cable, clutch, flywheel, shifter and driveshaft.
Prices depend on where you live. I have heard some say they paid as little as 75 bucks for an m46. I just bought one with wiring, relay, crossmember and shifter for 150 Canadian (I was going from m47 to m46). It would cost you more because you don't already have a clutch and flywheel.
The only other problem would be the rear end. I think the autos have a taller rear end, so you would rev a little higher than a factory standard.

"I think the autos have a taller rear end, so you would rev a little higher than a factory standard."

Taller gears are numerically lower. The manual trans cars used a numerically lower gear ratio than an auto, therefore taller.

For '92, the auto trans was paired with a 3.73:1 ratio and the manual was paired with a 3.31:1 ratio.