So, first I'm just going to preface this by saying, I'm Jen, Dwayne's wife. I am a Certified Child Passenger Safety Technician... which means I teach parents/caregivers how to install/use their child restraints.
Seats can absolutely fit into the MS3 (we own one and have kids

)... it just depends on the child and the seat.
With a new baby, I would look at the Graco SafeSeat1 or the Chicco Keyfit30. They are both infant baby bucket type seats that only face rear and both will last your child to about 1 year old (with the Graco model lasting most kids up to 22 months of age). They both allow the handle to be left up on the seat when in the vehicle which allows more space for front seat passengers. They both also install easily and have built-in lock-offs. Instead of locking the seatbelt, you use the lock-offs and then you will have no tipping issues with your seat (common issue with rear facing seats and switchable seatbelts - which the MS3 has).
I would also look into getting a full featured umbrella stroller (Peg Perego P3 or Chicco Cortino are places to start looking). They push really easily (even with heavier kids). They both have a full recline for newborns to lay flat, allow a carseat to snap onto for quick trips, nice big hoods, storage baskets and they fold up *small*... great for trunk space.
This is a Peg Perego P3 with a Graco Snugride attached to it...
Once baby outgrows the infant seat, you'll want a convertible seat... one that rear faces and then forward faces. You'll want to look for tall top harness slots so that it will last until a good booster age (5 - 6 years old) and a higher weight limit (50 - 65lbs).
You'll want to keep your child rear facing as long as possible (per current AAP recommendations). Children that are forward facing before the age of 2 are 4 - 5 times more likely to be injured in a collision.
www.car-seat.org is a great website full of techs to help you find the right seat for you budget, child, and vehicle. They can also give you more info on what is safest for you precious cargo.
This is a Britax Marathon in the MS3 rear facing behind the passenger seat. Once babies have good head control, the seat no longer needs to be at a 45 degree angle and can go as upright as 30 degrees. This one is installed at about a 33 degree angle and there is room for me to sit comfortably in the front (5'7") and room for dh (6'3") to sit in the front for trips around the city.
Britax Marathon rear facing with my 22 month old in it (26lbs, 35" at that time)...
Please note that this seat is tethered rear facing and is one of only two brands on the market that allow this to be done. Please do *not* tether your seat rear facing unless your owner's manual states it is allowed.
The seat in the foreground is a Sunshine Kids Radian (totally made for this car when forward facing). Here's a better picture of it...
Okay... if you made it this far through my book, Congratulations! I hope some of that info is helpful to you.
Jen, CRST (Dwayne's wife)
ETA: The middle is the safest spot IF the seat fits there properly. If it doesn't install well, then a proper install in one of the outboard positions is safer.
Seat protectors are *not* okay... they change the install and give you a false sense of a good install. A *thin* single layer towel is acceptable to help protect your seats.
