Here's a list of some of the things I can think of -
Wiring loom
Instrument cluster
Battery and possibly battery cables
Suspension springs (fronts and possibly the rears)
Exhaust system
Fuel system from tank to engine
Aftercooler and possibly the radiator /cooling system
Engine ECU (as you thought)
Webasto auxiliary heater if the diesel is a pre 2003 model
Turbo wastegate and egr control valves plus vacuum pipework
For a more detailed assessment use the Vagcat parts catalogue to compare petrol and diesel models -
http://www.vagcat.com/epc/
Cheers Spike