Late to the thread, but I thought I'd add a couple of thoughts:
(1) For cargo carrying with a rear seat, take a look at the Easyfit XL adapter, which allows you to have rear panniers along wih a Yepp Maxi or Junior seat: Home / Bicycle Seat / GMG Yepp Bicycle Seat / GMG Yepp Parts / GMG Easyfit/Junior Parts / Gmg Adaptor Easyfit Carrier Xl It would put the weight fairly far back but may affect handling less than a front rack.
(2) I recommend ordering kids seats from Holland Bike Shop; even with shipping they are substantially less than U.S. dealers
(3) Get a double kickstand for ease of loading. I recommend the Ursus Jumbo.