I have a 1988 300TE and I have a good mechanic. However, much difficulty getting the AC to function properly. I wish I could give more info, but I am not technically oriented. Could someone please advise as the what might be wrong and what to check first, last, etc. Thanks.

Most 124's have problems with the evaporators and compressors. If not diagnosed properly may lead to other problems and much $$$$. You did not say what kind of problem you have, or what symptoms, but take your car to somebody that knows MB AC systems.