search and ye shall be rewarded. all these questions have pretty much been answered I would guess, and the built-in search or google will point you to the relevant posts.
--having said that, and welcome, your commute should be fine as long as the roads are paved;
--not familiar with the size of the boston terriers in questions, but you and human #2 should fit comfortably and there are dog owners as you can imagine on the forum here that can attest (sometimes with pictures) to the suitability of smart canine transport;
--service, at least through the warranty period should be by the dealer;
--smarts are required to be sold at MSRP, all additional charges being options, fees and taxes. There's a thread here about dealer fees, you can look at your dealer and see how they stack up. dealer fees are usually state-by-state;
--mpg greatly depends on driver style, and environment being driven, but figure 30 - 50. if secondary roads means <=55mph, and little or no stop and go traffic, you should be good to go for 40s.
--as far as problems, best to spend some time looking through the forum, that's what it's there for. all I can attest to personally is that there have been no problems on mine.