I would personally opt for the infant seats. But we got a lot of mileage out of them (16 months or more as my girls are tiny). If you have any friends who could pass theirs on to you that would be great. Otherwise, buying used ones from a reliable source would be cheap. Personally, we are always happy to pass on our things for free to others, so hopefully you could find that situation by you.
We had a Camry, and we had intended on keeping that, but it died, so we ended up with a minivan. Convertible seats are big, but if you got those expensive narrow seats, you could put them possibly in the middle and behind the passenger seat. Radian..... I think that's the name of the narrow seat. Good luck!