LOL, more seats will fit in there than you think! I have an '05 Cooper S. What exactly are you looking for? For car seat, are you starting off with the infant seat? Who usually drives the car and how tall are you/dh? What style stroller are you looking for?
If you're starting with the infant seat, I've had the Snugride 35 in there, but it's not ideal. Best bet is Cybex Aton (if you're in the US), then the Keyfit. Handle can be in the upright locked position for these seats, so it gains you a ton of room. I can tell you about convertibles too if you want.
Strollers? I've had a few in there, the only one that truly fit well was the Quinny Zapp, but I hated the stroller. I had our Phil and Ted's Sport in there as well, but it took up space. I like our Smart stroller in there, even though the fold is flat, I can still fit some stuff in front of it. I also wanted a Mamas and Papas Urbo, it would have fit as well. The traditional Graco style strollers won't fit, at all.
Apparently the Bugaboo Chameleon fits, but I've not seen that in person. Also try the City Mini, Bumbleride Indie, Baby Jogger Versa, and UppaBaby Cruz. If you can try them out in the boot then you'll get a good idea of what fits.