A used Rialta would make more sense to me; much more secure, you would have holding tanks and everything that IS going to have failed/fallen off will already have done so. Plus of course you wouldn't have to deal with a VW dealer who may or may not provide warranty service for the chassis.
Look into snagging one of the 5cyl MB diesels in a Sprinter-all metal and the engine is a work of art. We averaged 28mpg all day-lost the blower just outside of Detroit and were getting over 31 mpg but of course that was on the back roads and there were some hills that we barely got up....turned out to be the vacuum control module.