Thanks a lot for all the advice! Really valuable nuggets in there.
We don't plan on full time living in this thing. It'd be weekend trips here and there. I really liked the idea of a self-contained platform that we could just pack up and go since prep time is less trail time. However, you all's insights have made me re-evaluate my expectations. I originally had the towing expectation for a small SUV so my wife could get around when she wanted or so we had a spare set of wheels to not have to break camp anytime we wanted to range out somewhere.
Martin - I think you're absolutely right, going with a single sole platform does ask for a lot of compromises not just of the vehicle, but of my family. You also hit it on the head about keeping the wife happy. I just don't think it'll happen with a single platform, so the trailer combo is now a serious contender.
Carleton/billiebob brought up a great point too with safety belts and seating. We have one in diapers and the other a toddler, so I need to be able to put decent car seats in. Grandpa/grandma may come out with us as well, which means seating for five.
Taking all this into consideration, I'm leaning towards modifying the Ex a bit, along the lines gatorgrizz/shovel mentioned - suspension, raised roof, bumpers/winch, self-recovery gear, internal storage (gear for outdoor kitchen/shower/storage) to make it a fairly capable overland truck, but really only need to provide some sleeping space in it if I'm going to venture farther from a "base camp" with one or two of the kids (obviously when the infant is older).
I haven't eliminated any of the other choices just yet though. So far, I think a 4x4 econoline with the right mods can probably do all the off-roading I'd want, and ultimately have more interior room than the Excursion. It could probably have similar towing capacity as well. I just think it'll cost a bit more to get a van setup properly and I'd have to go through the process of selling the Excursion.
A transit van may still work too, but I definitely would stick to an EB size due to the overhang issues Carleton mentioned, and am aware that they will always be the least off-road capable.
For the folks mentioning FWC, I took a look at this
video/site which I thought brought up a couple good arguments why the
van option is better. However, I'll be honest, I have't researched FWCs all that much so I'll take a look and maybe they can beat the camper trailer option if they can be dismounted and function as a standalone for basecamp.