If that really sells for $15,000, I'll be amazed. Especially in this economy.
I passed on a very similar, recently surplussed, Fire Dept Unimog BOV for $1500 at auction. The interior wasn't quite as polished though.
I think you could build an identical vehicle for much less money. Similar trucks (not converted) at auction, are rarely going for more than $2,000 here in Albuquerque.
The "26 gauge aircraft aluminum" is hype. That is 16/1000 of an inch, easy to dent with your knuckles, and aircraft use a variety of aluminum alloys, some very soft. I doubt he has any idea what alloy his is (Home Depot's best!).