Shaking engine at idle- very smooth at slightest acceleration

[86560SEL]

I just posted this on the diesel forums and have gotten good response, but want to get more input on this potential problem.

I have found a local 84 300SD w/330K miles in excellent condition (appearance wise) for $2399. Same owner since 1989- garage kept- white w/ gray interior.
The possible problem that has me concerned is the severe shake the engine has at idle. It will actually shake the entire car. Just the slightest acceleration- the engine will smooth out as smooth as silk. Its only at idle when it does it. It has 330K and the seller stopped driving it last summer because the A/C went out. It has been in the garage for a few months. He said it was not doing it before and it may need a "tune-up". I know nothing about Mercedes. Could this be something as simple as a tune-up, or is it something major? My dad said it sounded like an injector is going bad. I really like this car, as it is in excellent condition and if it is nothing major- I will probably buy it. I am supposed to go back tomorrow in the daylight and drive it. I wanted to check here first to check on this engine problem before I bought it. BTW- he is asking $2399 for the car, but I think I can get it for less than that. ANY advice/information greatly appreciated. On the other forums- someone said it could be a weak "rack damper bolt". Others said it could be a clogged injector or motor mounts. It has a "put put put" sound from the tailpipe- similar to a miss. I seen another topic about this and it turned out to be engine shocks.
I just want to be sure it is not a major problem before I buy it.
ANY advice appreciated.