Great! Thanks for the answers to the questions!
It sounds as if your daughter has longer legs than torso, which for carseat use is great! It also sounds as if you don't need to worry about fitting her seat next to another seat at this time, which is, again, great!
Because your daughter does have long legs, I would avoid seats that are known for having little leg room. I would also hesitate, due to your daughter's low tone, to recommend seats that don't have a lot of lateral support.
I think the [ame="http://www.amazon.com/Size4Me-70-Convertible-Car-Seat/dp/B0092KT2S0/ref=sr_1_1?ie=UTF8&qid=1376355395&sr=8-1&keywords=graco+mysize"]Graco MySize70/Size4Me/Headwise [/ame]might be a good option. It is a big seat, so it will look as if it is swallowing your daughter right now, but she will quickly grow into it.
It has a lot of support for her torso and head
and will last a VERY long time rear-facing. It is a seat we generally recommend to parents of long-torsoed children, but I still think it would be a good option for you due to its longevity and support.
I also think the [ame="http://www.amazon.com/Chicco-NextFit-Convertible-Seat-Mystique/dp/B00AHPZY68/ref=sr_1_1?ie=UTF8&qid=1376355328&sr=8-1&keywords=chicco+nextfit"]Chicco NextFit [/ame]might be a good option. It too is a big seat that might look overwhelming at this time, but it won't for long. It too has a good amount of support for a low-tone child and will last a very long time rear-facing.
There are many other options, but the two above would be my top-two recommendations for your situation.