Personally, I would build it myself. I can't say this would be cheaper, but it could save you time & trouble later on. I have saved thousands of $$$ and countless hours setting up my own rigs. For what it's worth: Say you decide to expand (or narrow) your service offerings (decks, flatwork, siding, etc.) over a period. If you purchase a pre-designed rig, it may not fit your needs a year or two down the road and you'll end up changing things, or have to work around a layout that's not efficient anymore.
We kept our eyes open for used, enclosed trailers, hot water rigs, etc. My favorite rig cost me around $7,000, and I spent a year using it before we "tweeked" the layout to fit our needs. 14' tandem trailer, 1 yr. old hot water skid unit, water tank, etc. Our experience was that what we thought might be the best layout changed over time as we used the rig out in the field. Of course, you could change things around if you bought a pre-designed rig, but you might cost yourself a lot of innefficiencies over the first year or two. Good Luck!
I thought you meant a trailer rig set up. The only thing I'd question in building your own hot water unit is the frame, or skid portion. The engine, pump and heating unit should be fairly simple to piece together, and the individual warranties should apply if something goes wrong. My feeling is that the frame portion is going to be important as you have a lot of vibration. You're the one who needs to decide what your times worth in putting the whole thing together. Also, if you have a dealer nearby that you would normally purchase a rig from, you may getter better response and lower, or little repair costs if something goes wrong. A dealer is going to have a lot more pull in getting parts and warranty repair approvals (reimbursement), than you might as an individual. Good Luck