Go to www.car-seat.org, post your questions just the way you did here, and the technicians there will give you the answers you are looking for... or guidance to keep you and your child safe. By "technicians" I mean these people are trained, tested and certified to safely install carseats. Because of what they do, they have a wealth of knowledge about cars, carseats, and what size children will best fit and have the safest ride in their seat. Also, most of the technicians are moms, so they can also offer advice as to what they use for their own precious cargo.
I was stumped when I was seeking the next step for my 4 yr old, 40 pound son... with two completely different vehicles, and they advised me wonderfully. Post with them and you will feel better, trust me!
A tip that I got from them is to keep you child rear-facing for as long as possible... even beyond what the guidelines indicate. It is the safest place for your child to be... and a five point harness (now and when she's bigger) is safest for her too!
You'll do great!