Building a propane burner for an exterior smoke box

[brujito]

I am considering starting a small catering business to make extra money over the weekends during the summer and early fall.  I am looking to build a trailerable smoker from a decommissioned propane tank with a separate and detached smoke box that is heated by a propane burner rather than coal.  Could anyone give be some advice on what would be the most inexpensive route to build a functional smoke box and a proficient burner?  Also how large of a propane tank would i need to smoke for 8 to 12 hours straight?