Welcome aboard.
To state the bleedin' obvious, it sounds like you've got a dead BW35 if it''s not going into reverse. Do you plan to keep it as an auto or are you thinking of converting to a manual?
Loud rattles in the nether regions can have many and varied causes. B20 engines are incredibly tough, and 124k km is usually about one-third of the distance done before the heads are pulled on these engines.
The fuel injection system on a 1974 140 should be the K-Jetronic system, AKA Bosch CI. There are some tricks to that system, but mostly if the guts of it are kept clean and the seals are good, it will go forever. One of the tricks is in the fuel distributor, as
@bigal shows in page 2 of
this thread.
Poor idle can have any number of causes. Plugs? Leads? Timing? Fuel pressure? The list goes on.
Anyway, there's a quite a few of us with '74 model 140s on the board, and we'll all be interested to see what you find as your investigations proceed.