Does the tach work? There's a dial by the brake booster marked 1 through 6. To change the setting, pull out the dial, turn to a different number, press in the dial. Higher numbers increase idle speed in [IIRC] 25 rpm increments, lower number reduce idle speed. Going from 6 to 1 drops idle by 150 rpm so there's no risk of 'over turning' the dial. More than anything, does the engine respond to different dial settings? If so, new engine mounts go a long way towards smoothing the engine at idle. In some cases, so does an Italian tune up.

I'm not familiar with the #16 casting since it isn't in MB's upgrade path for the turbo 603. There is history to suggest #17 and later castings aren't prone to cracking. I don't know where the #16 casting came from since according to the US EPC it was never available in the US.

$2500 should buy a car that runs properly.

Sixto
87 300D