Your 124.128 300D CAME from Sendelfingen with a 722.418 Transmission.
Reverse difficulties CAN be symptomatic of a DECLINE.
FIRST...Have you "Serviced the Transmission", NEW Filter AND drained and refilled BOTH the Pan and Torque converter?
Howeveah, The 722.3XX series Tranny is THE upgrade
(You'll blow your 2.5 up developing power ,before you hurt a 722.3XX)
member ps2cho has recently rebuilt his 722.3XX and all the details are here:
http://www.peachparts.com/shopforum/showthread.php?t=271601&highlight=transmission+pump
The ATSG is a good starter.
(SOME old hands believe both the .3 and .4 were originally GM designs,purchased by Mercedes,in days of Yore.)