I think that although retro-fitting of these items may be possible, if you're talking dealer prices (which you'd likely have to do to maintain your warranty) you'll need pretty deep pockets! A couple of weeks ago, I managed to get a Fiat radio with DAB and have fitted it into my 500L, but if you look around, the standard non-DAB radio's (if you can find one) are going for anything from £225 up to around £350 so what a dealer would want for the included Sat-nav version is anyones guess, but I doubt it would be cheap! Seat-wise, I reckon a dealer would want to swap the seats rather than fit heated seat pads into the cushions, so again I think you'd be looking at mega-money. As an alternative, could you buy heated seat pads, unbolt the seats and find a local upholsterer who could strip and fit the pads into the seats for you? You'd then just need to wire them in (if your car didn't have this option when new, wiring may not be already in the cars harness). Even after the expense, it's doubtful if the car would be worth any more money when you eventually came to sell it, so you'd really need to get your moneys worth out of it before selling the car on.
Keith.