I've seen some really cool rescue rigs, but they offer some rather unique challenges. . .
While they've got plenty of storage, it's all on the outside - and that storage sticks into the inside of the "passenger" compartment on the back. I could see a cool build with a bed, and a kitchen, but I'm a little stumped as to where I would put the toilet/shower. (The only full-height area is the walkway down the middle of the box in the back.)
I love that you get a lot of truck for the money, you just don't get that much interior space. You could weld the side compartment doors shut and cut out the cabinets on the inside, but at that point the "rate of return" gets too high for me - more work then benefit.
I've also considered one because it shouldn't have the insurance issues that some people have trying to convert a bus.
I've even considered a step van conversion, but good ones that aren't terrible/scary on the highway are expensive, and the bus options have them beat.