short of swapping a 55k motor or supercharging the W210, theres not really a whole lot of bang for the buck mods on the W210.
As for making it handle better, your working with a luxury barge (not sedan lol), and your fairly limited in terms of what you can do. I do know they make aftermarket swaybars, bushings, and coilovers for the W210, but like I said before, its a barge compared to an STi.
With that said, anything is possible.
Thats a friends 03 E55k being put through its paces on the track. While it may not be a lotus, or a Porsche, it is still a beast!