New infant seat and seat for 4 year old.

I just discovered my daughter's marathon expires a good 6 months earlier than I thought. Long story short, we bought it from a great local place now gone and for what we paid it should have been newer. So it expires in 9/12 so sad since we picked it up full price 10/07. I now need a new seat and if anyone can tell me how close I can get to the expiration date safely that would help. The only place where you could buy and have seats installed went under (same place) so I can get it installed but not where I buy it.

Child 1 is 4 (will be 5 in October) FF in a marathon at the moment.
She is 35 pounds, 42 inches tall with a 14 inch torso. I am having trouble as she gets older getting her to sit with her but against it and getting her in and out when she is not cooperating is getting harder for me. Are we at the size point where I could consider moving her to a harness seat that becomes a booster like the frontier or is it too soon? My other concern is that she is now in the middle, I would prefer to keep her there as she is not rear facing.

Child 2 is coming in late august, early september. I need an infant seat. My preference would be to keep #1 in the middle and put #2 on either side as it will be safer for #1. Is that a good idea? Are there seats that work well together or should I do each on their own side?

Our car is a 2003 Ford Focus with latch. We need 2 seats, price is a factor but safety is more important.

You can use the Marathon up until the expiration date (6 years from the manuf date).

Keeping the kids side by side is fine as long as you can find seats that work (and it will probably be helpful having the older child to entertain the baby - doesn't always work so good when the kids are closer in age - easier to hit/kick/bug each other ) Even though you have Latch it might be easier to get the seats next to each other if you use the seatbelt, that way you can move the outboard seat closer to the door to leave more room for the seat in the middle. And you want to make sure the seats aren't relying on each other to stay tight (what we call independently installed). To do that install one seat, then the 2nd seat, then take out the 1st seat and make sure the 2nd seat is still tight.

Now for seats to look at. For your DD, the Frontier is a great seat for someone her size/age but it is large and you'll probably have a hard time getting another seat next to it. The Maestro (evenflo) is a narrower combo seat (harnessed/booster) that would most likely last her until you're ready to move her to a booster (age 6+). The other option depending when the baby's due and how long baby might fit into an infant carrier would be to get a Radian for DD that could be passed to the baby and DD moved to a booster at that time (if baby will last for 12-18 months in the infant seat this might be a good option - kind of hard to predict the future though).

I'm a little out of the loop when it comes to infant seats. The Graco models, Chicco Keyfit & Safety 1st Onboard are probably the most popular recommendations. The Keyfit & Onboard are probably the narrower options and of those the Onboard will last the longest (taller shell).