I had the "reverse only" problem last month also.
I have a W123 280E and the reverse band had broken.

Your right about Sam at Khoury Spare Parts. As MB owners
in Melbourne we're fortunate to have him there.

I repaired the auto myself, and it cost me around $600 in parts and a lot of stuffing around being the first MB tranny I have worked on. The rear band itself cost $500.

My understanding with the transmission in my car (It is a 722.3 type auto) is that they have a weakness with rear bands and rear servo pistons.
Normally in local cars exchange bands are available, but with this auto no exchange available as they all break! I am led to believe
that the new bands are stronger than the old type...I hope
this is true!