We've got a 5 y/o, with another one on the way. For the booster seat, we've got a Britax Parkway that fits beautifully; I believe it is also available in the US. He fits in the car great, and really likes riding in it.
You guys can't get the Quinny Buzz? We can get it through Sears, but it is an online order. Our Sears has the Quinny Zap in store, but I don't know if it fits or not.
For baby car seats, we've found that the Italian brand Peg-Perego seems to fit our
MCS the best. They also have a wide range of strollers (that also all fit the baby car carriers), the smallest of which doesn't
quite fit in the boot, but it is close. Their system is pretty cool, you purchase the car seat and stroller(s) separately, so you're not married into one combination.
You always have the option of having a roof rack carrier! Or if it is only mom or dad in the front, just fold up the mall stroller (with seat attachments) and park it in the passenger seat well.
Having said all that, we also own a full size SUV for larger family outings / vacations.
A MINI as a second vehicle should be more than doable!
