Yes, get a Marathon (or other Britax 65 pound weight limit convertible) if your budget and space allow it.
Babies can be more upright than 45 degrees when they are older with better head control, so keep that in mind. Also, like Tara said, Britax seats are allowed to be braced against the front vehicle seats, so you can install the seat tightly where you want it and then scoot the front seats back to push against the top of the carseat shell.
If you can't go with one of those, I would go with a Cosco Scenera over a Roundabout. Even though it's much cheaper and doesn't have as much padding or EPS foam, it has a taller shell so kids can rear-face in it longer (rear-facing is much safer than forward-facing, and studies show kids up to age 2 are 4 times more likely to die in a crash if they are forward-facing). It has a 35 pound rear-facing weight limit (Britax is 33 pounds).
Forward-facing the Scenera (or any 40 pound weight limit convertible) will not last nearly as long as the 65 pound weight limit seats, only about 2-4 years instead of 5-7 years.