I have a 2003 C320 Mercedes & am needing to replace the front 2 tires & rims. I have 255/35/ZR19 on the back so would a 245/45/R17 work for the front?
In my experience tirerack.com does a fantastic job of determining appropriate fitments for all kinds of cars. They do list some 17" wheel fits for the 2003 C320, but they suggest a 225/45-17, not 245/45-17.
Your current rear wheel/tire setup is a bit bigger in diameter than their recommended packages. Their recommended 19" fitment is either 225/35-19 all around, or 225/35-19 front and 265/30-19 rear.
The front setup you've asked about is also bigger in both width and rolling diameter than anything I've seen recommended, which probably doesn't bode well for a good fit--I'd expect you're in danger of rubbing the wheel well.
If you've got access to the wheels and tires you're talking about, it can't hurt to try bolting them on, but based on what I've seen, I wouldn't order/buy that size for that car. I'd stick with 225 wide for the front.