The infared gas heaters kick ass for sure and that's one way to go. The radiant heaters are another option for a fully finished and insulated garage like the one being discussed.
As smart as some plumbers are, talking to them about heating is like talking to your electrian about circuit boards and controls. Installing and fully understanding systems are two seperate deals. I'm not a sparky or a turd hurder although I do and have done both more than I've wanted
If it were me, and I had this guys set up, I'd intall two of the radiant tubes and be done with it. Gas already on site and building already insulated, bing, bang, boom.
Dan