You already got some great advice. I'm at work so I can't post my full out "how to find a convertible seat" post right now... BUT... you need to DEFINITELY find a convertible seat...one that rear faces until a certain weight limit and then forward faces to another limit.
The best car seat for your child is...
1. One that fits your child correctly.
2. One that fits your car correctly.
3. One that you will use correctly EVERY SINGLE TIME!
This means that the straps are easy to loosen and tighten EVERY time you put your child in and out, etc.
If your child has a long torso, you want to find a seat that has higher harness slots rather than shorter ones.
A higher weight seat (like the Britax seats, the Cosco Apex and some of the other newer seats) although more expensive now...will give you more bang for your buck as you will be able to use the seat longer and possibly skip purchasing a combination seat later on.
I don't have a lot of time...but I did want to add a few notes:
1. All children will outgrow their rear facing seat once they either hit the weight limit of the seat OR the top of their head is within 1" of the top of the shell (the hard plastic part not the padded cover) of the seat.
2. All rear facing seats are designed to attach to the car near where the child's feet are...with either lower anchors or the seat belt. This will allow some rebounding from the opposite side of the seat (under their head). This is perfectly normal and will cocoon your child in a crash situation. Britax does allow you to tether their rear facing seats, they use a bit of different technology.
3. DO NOT use aftermarket products in or with the carseat. Extra padding, harness strap coverss, rear view mirrors, towels or seat protectors under the seat, pull down sunshades (window clings are fine) etc. These may interfere w/ the way the car seat works (voiding the warranty) or become projectiles in a crash situation.
4. Get to a CERTIFIED car seat check station and learn how to best install your seat. Everyone calls it getting your car seat "checked" but in reality you don't just want them to check what you did, but teach you the best way to install it yourself. You won't have a tech there to reinstall it for you after your child pukes all over the seat for the first time! (Unless of course if you live next door to me
). When I get the chance I will list a link for you to find a local station where you can go.
If you have any questions just give me a yell
Oh and both of those links are invaluable resources containing tons of great car seat info!