way2roll,
That's a pretty ambitious project, and considering you don't have the space available to work on it, I wouldn't even consider it.
That said, your friends "custom Sprinter"....has he brought it to the scales to have it weighed? I've read many things about the Sprinters, and a common issue is that they are very low on available cargo carrying capacity. If your friend built it like he'd build a house, without any consideration for the weight of the materials used, he might be VERY overweight. In my opinion, that would be unsafe. Again, in my opinion, that would quickly take the bloom off of THAT rose! LOL
Obviously, if you work on converting a larger chassis which has ample weight capacity this probably won't be a concern. But then we're back to the lack of space.
In my opinion, and what I'm thinking about, is to pick up a nice, 20 year oldish , low mileage, well cared for, high-end Prevost or similar when I'm ready to retire and do some serious travelling. Personally, I'm not worried about the "green" thing, but maybe you can convince your wife that by purchasing a used motorhome, you're actually doing a GREAT thing by getting more life out of everything that went in to create it. The ULTIMATE recycling. You're not using ANY new materials, unless you make some mods to the existing layout.
Good Luck, hopefully things won't get too heated! :)
~Rick