Hi Phil,
I've just moved from an X250 S Portfolio to an X260 S with rear climate control (but sadly, no chilled front seats!).
I would imagine that retro-fitting rear climate stuff would be a bit of a mission - vents in the back of the centre console, with associated trunking, vents on the door pillars, also with associated trunking, plus fans and a different control module / screen in the back!
Personally, I'd prefer the cooled front seats (only two of us) but when there are four on board, the back row love the cool breeze &/or heated seats.
I wish you the best if you go for it! Let us all know how it goes!
Cheers,
Jon