Yep, 240 was the 4cyl, 300 5cyl. The latter was NA briefly before it got the turbo OM617. That turbo motor originall came in the old W116
chassis S Class car, which was a fine car in and of itself other than maybe the climate control, and US bumpers.
W123 chassis is close enough between the 4/5cyl that one can swap em around without too much grief.
I'd actually like to rebuild and turbo/intercool the little 4, it was done back in the day and I know stock 617 turbo
power can be had from it with reasonable lifespan. There is a very pleasant amount of room under-hood
with the smaller motor in the w123. People use the 240D 4spd manual with the 617, there are technically some balance
issues, and in fact the service manual says not to do it interestingly, but a lot of folks have now. The euro manual 5cyl flywheel
is unobtainable, much like the actual 5spd. Less known is that the pumps are calibrated differently for manual/auto. Having a stick
on an auto pump'd motor feels "weird" but you can get used to it, the mile of crazy German throttle linkage isn't helping matters
but again time breeds familiarity. All those old Mercs were that way to a degree, lots of throttle travel, you really had to foot-down
on them. My last daily Merc was a 73 280 gasser with the dual cam M110 I-6, carb'd with an exhaust and euro manifolds and a tweaked
carb, fun car if not overly efficient. The W114/115 is a better chassis in a lot of ways than the W123 in my opinion. And dirt cheap.
Getting off track I know, but another thought has been that a Volvo 240 wagon would be a good place for a turbo 617 to live.
I appreciate the diesel they were available with, but they are scarce and expensive to repair relative to the Mercedes motor.
I think that'd be a good driver after it was sorted out. There was a kid on turbobricks that put a TDI in one awhile ago
with great success.