2 of my friends with special needs had the
Sunshine Medical Kid Kart.
It worked well for both families and their girls who needed many of the additional straps and supports. Both girls are tiny, so they used it from age 2 until at least 5-6.
Fits in the trunk (even a Civic), has an "indoor" base as well as a stroller base, room on the bottom for "stuff."
I'd recommend getting the "transport" option so the chair can be used on the school bus. Even if you don't plan to use it, it's better to have it and not need it than want/need it for even occasional school use - think field trip if not daily transport. [It's not the 1st choice, but for kids who are non ambulatory, it's often the only choice once they reach 30-35 pounds. That is the weight limit for a "1 person lift" in my school district, and they will no longer shift in/out of their stroller or w/c into a SN seat. Liability.]
One of the girls now has this:
http://www.wike.ca/special_needs_large_bicycle_trailer.php
Great bike trailer and a VERY nice stroller for a big kid (12 yrs, 75 ish pounds) who needs support.