Thanks for the input gents! IS3 is for sure not well equipped until you get to the Premier model, which is what I would look for. And tho that model has ML as standard (I think), few also have the moonroof, both of which my IS2 has.
I think I'd be lucky to find another, newer IS2 with those extras, that has a complete dealer service record from new, as mine does; and if I did it's likely, as Linus points out, to be overpriced. Besides which, I am slightly superstitious about replacing like with like. I've done it twice before, both times a disappointment.
So better the devil you know, I guess?