Third set of mounts but still the original shocks?
Seems odd to have the original shocks and be thinking about a third set of mounts, but maybe I misunderstood. I replaced my shocks and struts at about 75k with Bilstein Sports. Did the rears myself but had a shop do the struts. I like them, except when driving on really bad roads, when they do bump you around a lot. I also replaced my rear shock mounts (even though the original ones looked fine) and added the reinforcement plates, which you should do it you go with the Sports. And don't worry about the Sports being for cars with lowered springs; Z3s came with lowered springs from the factory.
Phil '97 1.9 Montreal Blue