Most interior stuff can be retrofitted easy enough, so having heated leather seats isn't that hard to achieve.
A heated front screen could be retrofitted too, if you don't mind spending on the screen itself. Incidentally only the TD4 and KV6 were ever available with the heated screen, it wasn't an option on the 1.8. I suspect this is due to the higher output alternator required for the screen, which the 1.8 didn't have.
I don't see this being an issue though, as the screen times out after just a few minutes, so the electrical system should handle the additional load ok.
You might have to play with the software on the CCU however, as it needs to be told the HFS is fitted, as it's all under CCU control.