My car starts out in second gear too, but there is no delay. That is normal. It sounds like it could be a timing problem though, but the fact that it does it worse when the ac is on, that throws me off. The increased drag on the engine should not have anything to do with hesitation, and the idle control valve should compensate for the increased drag by increasing the idle when the AC is on. I don't know if that model SL has an ICV, but it is worth checking out. Those engines are bulletproof and will run forever, I am sorry that you have had these experiences with your car. I got mine about a year ago, it is a 96 SL320, and I have replaced almost every seal and gasket in the engine (myself). I would think that after having these repairs made that something was not put back like it was. In other words a vacuum line was hooked up wrong, a wire got snapped, one of those plastic vacuum lines could have cracked, who knows. It will take some time to figure out. I can't imagine how frustrating it would be to step on the gas and to have your car think about it first, that is really strange. Tell your mechanic about the hesitation issue and the fact that your idle is dropping with the ac on. Good luck!
Happy Benzing!
Brian