If you have keyless go already fitted, then its easy enough to buy the start button to replace using the key.
Have you checked your datacard to see if its already fitted? The button wont work unless Keyless is already fitted.
A replacement button is about £35 from the dealer, or one of hundreds for sale on eBay for under a tenner. Yes, they do work.
REPLACES PART NUMBER：2215450714, 33161207, NOTE: NOT fit for vehicles that not equipped with the Keyless-Go option, just only fit for keyless go vehicles. OEM Number: 2215450714, 33161207. 2x Carabiner Clip Hook Heavy Duty 25kn D-ring Screw Outdoor Lock Rock Climbing.
As for a reversing camera, if you want OEM, get in touch with Comandonline in Lightwater anf they will fit one for you. £449 plus fitting.
Retrofit kits for Mercedes Reversing Cameras We can supply kits for the ML, GL or R class with and without TV, so please contact us if your exact vehicle is not listed here. Note that we order these kits specifically as needed, so they may take a few days to arrive.