Best way too find them cheap is when they don't run and need 1 major component(engine,blower,or heat exchangers)..
A guy sold a van with a
Prochem unit in it last week here for 4,500(asking price).. That guy got a steal, I have no room for it or I would have bought it, and sold the van for 2500. So you just bought a $2000 truckmount.. All truck mounts are is a frame, all the components can be replaced(if you do it yourself you can save big cash).
I bought one last year (that I shouldn't have) that needs ALL of the heat exchangers($3,000), new water pump($800), and new engine($2700).. I went against my rule of 1 component, but got it at a good price($2000) So it will cost me all and all $10,000 for a bran new unit, after a lot of work for me..
One major component is what should be stuck to..
Testing heat exchangers would be #1 on my list, everything else is cheap.. Though I would also junk a engine, rather rebuild, or even buying a new carburetor..