1. It is not possible to update
2. Full link is a SWaP function (Software as purchase) so even in the headunits that support it, it needs to be plugged into Seat dealer computer and activated with a code.
3. MIB1 units (which are on the older cars DO NOT support Full link in any way
4. Retrofitting a unit is not cost effective and it involves a very complicated process of sending the unit away to be hacked with a custom firmware and also paying the dealer to removed component protection.
5. Pioneer and Alpine have some custom headunits specific to this car, but, as you guessed, they are also expensive...
Tl;Dr: Forget about it or buy a car that supports it