Hi Nick,
definitely worth driving an R56 and an R53 to compare them. I'm not on your territory having had an R50 One and an R56 Cooper but they are worlds apart. To be honest I miss the drive feel of the R50 like mad. And yeah I think it's the early R56 that had more build/mechanical problems rather than the R53. You're gonna have to try driving both as it might make a big difference to your decision.
Have you looked into whether you can retrofit heated seats? If you would really miss it it might be worth pricing if it's possible so you cna factor it in. Unusual spec is a bit of a rarity. I gave up looking for a sunroof and had to spec a new car to get what I really wanted spec wise!