You can buy extensions for dog-collars or harnesses that plug into the fixed part of the car seat-belt. For smaller dogs consider a little cage/carrier that can fit in a foot-well.
I have a 4 kg terrierest and her 45kg Rottie brother and both a very happy with the arrangements above.
On the broader question, I was going to suggest a SUV type as the higher rear-seating positions mean there is less strain or potential damage to adult backs lifting chairs and babies in and out. For the same reason, I was going to suggest a car with a boot door that opens on hinges to the side, rather than flipping up like a H/B, with the added advantage of a flat load area.
Hope all goes well and happy car shopping. Off the top of my head, the Suzuki Grand Vitara is an example of the kind of car I mean.