Parktronic can be retrofitted and for the 203 costs approx £1800.
Quickpark is a rear only dealer fit option again for the 303 is approx £500.
There much cheaper after Market options that work equally well..
Worth knowing quickparc is audio only.
Parktronic is installed in the factory as the wiring needs to connect from the back to the front of the car so the proximity lights can be connected.
Quickparc being only audio has a speaker located at the back.
May as well get an audio parking sensor unit installed with a non-MB merchant it may save you some pennys.