Make sure that it will acommedate rear-facing and forward facing.
Go with one that will let you have your child on harness from from 5-20 lbs minimum up to 65 lbs. Otherwise you'll have double expense later.
Ex. We bought a Britax Marathon (65lbs) going on 4 1/2y. It was a bit pricey, but she is now 5y, 38 lbs and plenty of room. Cost me $300. Hubby's car won't fit a Britax, so we had to buy 1 carseat for $90, and now we just bought a booster for $50. She is not comfortable in it, but that's what fits for his car and she doesn't ride long periods.
My inlaws bought a carseat (40 lbs limit) for $60. Their almost 4y has reached the height and weight limit. So now they have to buy booster ($100 - 200)
If its not too late, ask for one or money for one for Christmas.