I'll leave the tow vehicle question for others to answer.
To figure how much to save, you have to figure what you want to do with it as far as the buildout goes. Let's say your bus costs $3000.00 (not saying you will find one for that). If all you do is tear out the seats and use it as a metal tent, then save up about $4,500.00 so you can have the vehicle inspected and serviced before you start relying on it. Of course, it will be way hot in the summer and way cold in the winter. If it needs tires, add another $2,000.00 to $3,000.00. It starts to add up quickly. I'd say STARTING with $15,000.00 might get a fair buildout if you are careful.
There is a place about 45 minutes south of Tucson, AZ called Caballo Loco Ranch
where one can live during buildout and beyond.
There is another similar near Graham, TX just north of Possum Kingdom Lake. It is called Awesome Possum. This is a video about it.