As long as your current vehicle is in good working order, I'd vote for new carseats. You could easily replace all your seats with the cost of just one or 2 car payments. I have a minivan too and 4 kids and I understand about the seats not fitting thing. We have a Nissan Quest and the back bench is so narrow that we can't fit the same 3 seats across it that we can fit across our Focus - a Fisher Price booster, a Scenera, and a Radian. And the buckle stalks on the captains chairs are set so far forward that buckling seats securely is hard. So we can't have 5 in seats either. Pathetic. A pp mentioned the Radian. It is the skinniest convertible seat on the market. It does not convert to a booster but it does rf to 33/35lb and can be used ff up to 65 lbs. And while other forward facing seats are outgrown when the shoulders go past the top useable slots, the Radian allows you to go past the top slots until the ears hit the top of the shell due to its steel frame. My tall ds outgrew ours at age 6. It can be a bit tricky to install but you can get the hang of it. You probably aren't going to be moving it around much. The Scenera is skinny too but most kids will get too tall for it by age 3-3.5 and ideally you don't want a kid in a booster until at least age 4 and 40lbs. It is also a lot easier to not have to dig for buckles and be able to use a 5 point harness too. As far as skinny boosters go, the Fisher Price booster (only online anymore due to being discontinued) adjusts down the skinniest and can be made wider as the child grows. Only $30 with $5 shipping at www.albeebaby.com. Since it has been discontinued, the bossters are already going to be 1-2 years old. Since they (like most seats) expire after 6 years, you won't get the full lifetime out of it but at $30 no big deal. I would highly suggest checking out http://www.car-seat.org/ or babycenter's carseat message boards http://community.babycenter.com/groups/a7645/car_seat_que... as they have helped me many times and can also give you good carseat friendly vehicle suggestions. Wish I would have asked about our van before we bought it. Congrats on number 5!
ETA: I have noticed several people recommending moving more kids to a backless booster. Boosters with the back on offer A LOT more side impact protection. Seeing the crash tests comparing the 2 is pretty convincing. IDEALLY you don't want to take the back off the booster until you have to - either the child gets too tall or the vehicle has non-removable headrests that get in the way of using the booster properly. If the vehicle has no headrests, it is even more important to use one with a back as that back acts as the headrest and helps prevent whiplash and sleeping support. There are a small number of boosters/combo seats that do not have reinforced backs and require that a headrest be behind them (it would say this in the manual). I think those models are made by COSCO, EDDIE BAUER, and SAFETY FIRST.