The problem with some of the retrofits are the security on the modules (Component Protection). Most dealers won't remove this unless the part is new so the cost are ££££, there are third party companies that have access to ODIS to remove CP, but I would find this out first. As said above even heated seats are not cheap, you will either need to find the seats with elements already installed, or fit the elements yourself and stripping a seat down is not an easy job, and getting them to look right after refitting the leather can be even more difficult. You then need a wiring harness, as Audi don't have the wiring in place for missing options. Finally you will need a climate control panel, which need CP removal and coding.
Rough Costs of heated seats
Wiring harness £65 (kufatec)
Control Panel £100 (Used off ebay)
Heated seat element £80 each (approx new from Audi) you need 4
CP removal £60
Total £545 and that's you doing all the work yourself + coding
You may find some of the parts cheaper on ebay but you cannot fit used elements, as they are glued to the foam base & back, and impossible to remove without damaging them.
Not sure what year you are looking at but around 66 plate seated memory seats & folding mirrors came as standard.
