engine swap in mercedes w204 = you need the whole car as source to swap.
point 1 the all control units are tied to the CAN and are coded only to talk to each other - meaning you cannot buy a junk ecu and make it work.
point 2 the key module is the junction for all the control units
point 3 - the key is the master release for all systems to get authorization signal and cross over network traffic for the BUS.
point 4 - the body control units like the pump for the door locks and control for the interior lamps is tied to the rest of the car through the key module.
So basically you need to swap a whole car to your car - and after that you need to keep track of the chassis and manufacturing numbers to get correct spare parts for it,