How much did you want to spend? We have a Mini Cooper and we have a Quinny Buzz. It fits into the boot in stroller (seat) mode nicely, but its a tight fit in bassinet mode. It fits because it breaks into parts - seat, chassis, wheels. But its not cheep.

We went to a couple of baby shops and asked if we could try out different prams in the boot. That ruled out most of them pretty quickly.

Regarding a car seat, we found a capsule fits better than a convertible in rear mode. The convertible seats have a higher back which comes out more when in rear facing mode, pushing the front seat so far forward that for our car it seemed dangerous. If you want to keep costs down you could hire a capsule. I think the S&S Unity is slightly shorter in length than the classic S&S capsule - its got a higher weight limit too.
We also have a Mini Cooper, What capsule did you end up getting Leebee? We are finding the safe n sound Unity tricky to fit in well, they all seam to move around so much!
We have a Phil and Teds sport pram which firs really well into our boot, leaving space for a bag of shopping or two as well....we do need to take out the parcel tray and one of the wheels just pops off too... very easy.