Ok this could get confusing
Mk1 C5's all Have VAN/CAN multiplexing
Mk2 C5's & all C6's have CAN/CAN multiplexing
This is a mk1 VAN/CAN RT3 unit, It cant be interchanged with a mk 2, apart from its pysical size it wont talk electronically to a full CAN car anyway. RT4 is not available on VAN/CAN

This is what the RT3 & 4 units look like from a mk2 CAN/CAN car

The actual units of RT3's & RT4's look the same on CAN/CAN types its the operation, features & software thats different, these units cant be interchanged either.
The units comunicate with their screens via a fibre optic link.
Retrofitting an RT3 to a mk1 is deffo not feasible unless you had a complete donor car to swap bits off. Get a tomtom for £100 quid & a parrot handsfree for £60 & your sorted.
Messing about swapping SIM cards around out your phone & into the unit every time you go out in the car would be a bit of a pain if you ask me.
Cant really comment on the RT2 system as i've never actually seen one in a car or played about with one, only ever seen them in handbooks. they seem a bit stone age now anyway.
The Citroen,peugeot & fiat units are all the same apart from the manufacturer button has a different picture.