I can probably help you out there with the supply.
Alternatively Steve Krabbenborg aka MbenzNL may to able to supply & install if he's in the uk.
Installation is a fairly straightforward DIY job. once its installed, you need to pop in to your friendly MB dealership and ask them to recode the dash display to show the nav menu & allow use of the steering wheel buttons for nav functions. they normally charge half hours labour around £40 fo ra 5 min job with their star diagnostic machine.
parts required are:
1 x comand - varies from £600 or so from Ebay to around £1k
1 x Wiring adapter £20
1 x GPS antenna with appropriate square grey connector (alpine do one) £40ish
1 x map disk - £100 but poss free copy with the comand
1 x user manual - not esential but nice to have £15
you may want the optional aux audio in cable - about £12
DO you have an MB cd changer? if not you will need 1 if you want to listen to CD's & Nav at the same time as comand cant do both from it's CD slot.