I think this is very much a personal thing. Child seats fit in the back of most 2+2 sports cars just fine so I personally wouldn't make a move until he/she (or maybe even they?) are past the todler stage.

Of course, there are two other considerations: firstly you'll probably be lugging an extraordinary amount of support equipment (nappies, bottles, changing mats, clothing, pushchair, etc) around - can you fit it all in your existing car? Secondly, children are an expensive business (or so I am told) so money may be a factor - you could probably make a tidy little profit from selling the Z3, for instance.

Probably not helping - sorry!