as an a3 owner (2004) with sat-nav
would it be possible or impractical to buy another such unit
and fit it in my 96 Pajero which has a double din size space?
It would be possible as long as you can generate canbus and a speed pulse.
A canbus emulator would give you the required canbus, but you need to find the vehicles speed pulse and feed that tp the nav system, also the nav should be coded for an A6 so it uses the speed pulse.