Before you "wind up letting the Mercedes folks" swap your
transmission for an ebay unit, have a look on this site for
good independent mechanics in your area. You might get
a better deal and find someone to do other repairs to your
car. If you really like the car and it is in good shape, it might
be worth rebuilding the transmission. The ebay units are
totally unknown units and you take your chances.
(Funny how most pulled parts have low miles, usually less
than 100K.)
If it hasn't been done, you should check or replace your
timing chain rails. (Standard warning with these engines.)

__________________
VR
1967 250SL