Remember that with an infant you will want the ease of getting the baby in and out of the car without having to unstrap him/her (and waking them up), so you will definitely want a seat with a handle that just snaps in and out of the base (along with snapping into the stroller if you like). One big consideration for buying ours was how long our child would fit in it. All infant seats only go up to a certain weight then you have to purchase a bigger one (which does not come out of the car). We got a Graco system which said he could fit in it up to 30lbs and was just fine but our son is 9 months and already 23 pounds and we ended up with a bigger seat months ago because he couldn't fit in the seat with a jacket on comfortably. We did a lot of research and got the Britax Boulevard seat (great safety ratings) and it should fit him up to 65lbs. Britax also has another model that works up to 80lbs. Britax seats are very expensive but you can't put a price on the safety of our children, right?
On a related topic, the big thing now is making sure your older child is in a 5-point harness system up through 80lbs! Car safety belts have been proven to occasionally malfunction with just plain booster seats used with seat belts and have caused many deaths from ejections and in roll overs. There are sites all over the internet with tributes to lost little ones from this.
So much to think about, you are smart doing it ahead of time! I wish you lots of luck.