The only issue you will run into is that the Marathon will probably not fit your newborn until he/she is closer to 4-5 months. They have to be at least even w/ the bottom slots to use it (rearfacing the slots have to be at or below their shoulders). I know it says rated 5+ pounds, but I don't know what type of kid they think is gonna be 5 lbs. with a 10 inch torso. I just sat my 11 week old in her sister's (2.5 y/o) Marathon tonight (we are also passing her seat to the baby and getting her a Frontier) and she is maybe 1/2 inch over the bottom slots, so if I wanted to, I could put her in it right now. She is top of the charts, though, and is all torso. My husband says that she has no legs :)~ Anyway, there are a few convertibles that could maybe fit a newborn. I think the Safety First True Fit and Cosco Scenera, but I have never seen a newborn that could fit the Marathon. The Scenera is $40 so a lot cheaper than an infant seat, so you could use that until he/she is big enough for the Marathon. It doesn't offer a lot of padding, but probably neither did your oldest son's infant seat (I know my 2.5 year old's doesn't).
Also, are you planning on wearing the baby? That is another thing to think of when you plan this. You won't have the convenience of running in and out w/ the baby or having a place to put the baby when you go to eat. Not there is anything wrong with this, I wear my baby a lot, but it's just something to consider.