That same company offers a kit to mount a gasser BMW 6 speed to the OM606. Given the general durability of manual trannies, you would think a better donor could be found, one that could be easily found in boneyards. If I was about to spend $1600 on a kit I'd write them and suggest a much more common BMW gasser tranny in the US. I'd have to do some looking to determine what that might be.
I know that back in the oh-ohs the boneyards around here were lousy with E30 5 speeds. Manuals perhaps less common in later model Bimmers, not sure.
Adapter Kit for BMW ZF 53-BZ to Mercedes OM606