I happen to really appreciate having PDC on cars (never had a camera but anything to help see behind the car where I cannot is not a bad thing) but I'm not sure I'll retrofit anything with the Smart, so far it hasn't been an issue for me. Hmm, however, I do have this reverse sensor thing from the UK that we bought for another car and didn't install so maybe I'll try that...
If you want it just for parking in your garage close to a wall though, it might be easier to just get one of those little parking bumps or other parking aids vs spending more and installing anything into the car.