All right, I'm going to jump in again because this is a fun thread!
Until reading the posts here and doing a little subsequent research, I would have never guessed that a Sprinter would be within budgetary reach. However, it looks like a high mile (250k-ish) Sprinter can be had for about $12k with the inline 5 diesel. Great fuel economy, can swallow gear like no tomorrow, and supposedly pretty nice to drive. A high roof would make a big difference in livability IMO. If I was going to go the 'cargo' route, I'd highly consider this option.
How MUCH driving do you plan? Have you estimated your fuel costs? Let's just figure 20,000 miles over a years time. That allows roughly 400 miles a week. Your fuel costs could look roughly like this:
Sprinter: $4000.00 (20 mpg with diesel @ $4/gal)
1/2 ton Cargo: $4,667 (15 mpg with gas @ $3.50/gal)
older conversion van / worse case: $5,833 (12 mpg with gas @3.50)
In the above example, there is a difference of $1800 between the extreme ends of the spectrum over the course of a year. It seems like a lot, but you won't be able to build out an interior like a pre-built camper van for $1800 if you want to include full amenities. If you are fine to get by with a coleman stove, cooler, 5 gallon water jug, headlamp, and down jacket for heat then you'll be fine but if I was going to full time I personally would want a little more home feel. It may be worth it to throw fuel economy out the door if you found a rig that was really built out the way you think you might want and was deemed mechanically solid.
Another consideration is use of space. I've seen some folks throw together functional simple homebuilds, including putting in some IKEA cabinets and such, but often times the available space is not fully utilized. With minimal real estate to plan your new home in, you really want to make sure that items are well organized, accessible, and spaces designated for storage are designed to maximize actual capacity. There are even some factory camper van layouts that, while great for an overnight trip, just do not accommodate much storage space for longer excursions.
You've got a pretty great problem to have, and I think a lot of us are envious of it!