D.M.
Definately get a convertable car seat. But you MUST keep it rear facing until she is a year old. My sons were both over 20lb by 9 months but a babys neck muscle don't fully developed til one year so they must stay rear facing. You may want to consider the new seats that go from rear faceing to forward to booster seat. The cost a little more but are cheaper then having to buy a car seat now and a booster seat later.