Yes you can change the steering wheel, but only with another wheel from a mercedes vehicle. I believe the big SL wheel is compatible with the R170 chassis cars.
do a search on the popular mercedes forums!
otherwise No, you cant use an aftermarket wheel, it wont work, even if the hub splines are correct for mbz cars. most aftermarket wheels will work on 70's, 80's and early 90's mbz cars, but not the new stuff, it's mor to do with the blinker switches and not the airbag, at least this is what HDDP posted. To find out just tear into it.