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!