Instead of throwing $$$ in the black hole--a good diagnosis is in order. First ensure the engine does not have internal issues--run a compression check.

Leaking seals around the fuel injectors (that prevent air leaks) are quite common and will definitely cause rough idling. The seals are dirt cheap and R&R a set is an hour tops. Easy way to check--dribble a little water (from a hose) on each injector one at a time--when you find a 'leaker' you will hear it and the engine will change. You can use vapor from a handheld propane bottle just don't light the torch. When the vapor gets sucked in, idle will change.

Also, the 'standard' heat range spark plug is too cold for slow speed driving and tends to 'carbon foul'. Pull one out and see if it is black (fouled) or white/light tan.(normal)

All the ignition wire connections and spark-plug insulators MUST be coated with dielectric silicone grease to prevent electrical leakage. Also, the internal insulators in the 1950's vintage plug wire ends will fail from age and the spark will jump to ground. Change the wires.

ALL the vacuum hoses on the engine are prime candidates for air leakage. Change any flexible hose or fitting that has hardened (probably all of them German rubber is CRAP compared to US and Japanese stuff)

The fuel injectors and fuel distributor may be dirty or damaged. It only takes a little water to damage the fuel distributor beyond serviceability. Try running a can or two of BG44 fuel injector cleaner through the system to ensure the injectors are clean--or remove the injectors and have your friendly Bosch shop clean them real good--they will get crud in them that will mess up the spray pattern.