One thing that sinks the battleship of newbies in vans/Sprinters is coming in underfunded. You don't realize that driving 80,000 to 100,000 miles a year exacts a heavy toll on your equipment and you can, sooner or later, expect stuff to break. Given "Murphy's Law", it'll be the most expensive thing possible and about as far from home as you can be, too-- at a time when the business is slack anyway so re-building a crippled bank account is difficult at best. I hasten to point out that Murphy was an optimist.
Turtle's right though--if you get past the first year you've probably figured out how to survive at least, and the business itself isn't a bad one. If you get something going on the side-- I occasionally help a guy re-build offices-- the money from your side interests helps keep you going.