I would vote for the van. I know how much it ate at me when I was offered foster placements that sounded fantastic, but I couldn't fit in my vehicle. We had a minivan at the time, but bought our 15 passenger van a few weeks later actually. My experience was that going from 0-1, and 1-2 was challenging, but when we had 3 and went up to 5 it actually got easier, with the different ages we had (had a 1yo, 3yo and a 4yo who was developmentally 2yo, added a 6 and 8 year old.)
Anyways, I've had anywhere from 1-7 kids at a time at various points, and it really doesn't get much harder after 3. Just more laundry to do. Assuming that foster care per diem covers their expenses, and one of you is able to be a stay at home parent, anything is quite doable. We have a 4 bedroom house, but one bedroom we use as a playroom. We took the little bedroom for ourselves, and 3 boys in one, and 4 girls in the other, obviously utilizing bunkbeds. The 11 yo whines about wanting her own room, but she whines about pretty much everything. The rest do quite well with sharing rooms.
Anyways, I've had anywhere from 1-7 kids at a time at various points, and it really doesn't get much harder after 3. Just more laundry to do. Assuming that foster care per diem covers their expenses, and one of you is able to be a stay at home parent, anything is quite doable. We have a 4 bedroom house, but one bedroom we use as a playroom. We took the little bedroom for ourselves, and 3 boys in one, and 4 girls in the other, obviously utilizing bunkbeds. The 11 yo whines about wanting her own room, but she whines about pretty much everything. The rest do quite well with sharing rooms.
DH has decided that he is too old to have another baby--he's 46, I'm 37(almost 38). SO, I guess our family is complete with just one child. That's not what I wanted or had planned but that is what God wanted for me. Nothing else I can do about it. What we want and what God gives us are oftentimes two different things.
I really believe that parenting is different for everyone, like tastes in food, etc. You won't truly know what you want, enjoy and can deal with until you have some experience as a parent. I would have NEVER thought I could handle 4 kids. Heck, I didn't WANT 4 kids before our first was born. Now I can't imagine having less. And I swore that none of our kids would ever share a room. With 4 kids in a 4br house, yep, 2 are sharing. And it's okay. We do have one competitive gymnast who costs us a small fortune, but it's important for her (she has some needs that gym fulfills that we've never found a sub for) and we make it happen. Our kids wear good shoes, good clothes, and have really good car seats. We can't afford to travel like I wish we could, but sometimes priorities shift as you journey through parenthood, too. I can't imagine having more than 2 kids and NOT having a minivan or 3rd row SUV, but we like a vehicle with a lot of room (and we have a 175+lb dog 
so we purchased a minivan and a Yukon XL (gas guzzling thing that it is), because we tend to tote our 4 kids, their friends, groceries and stuff.
