It’s time to grab a good book on radiant heating and dig in. How many loops are we talking? Is there a manifold? Is this just one large loop?
You’re going to want to perform a leak test using air pressure. You can get the Schrader valve fitting, Pex rings and crimper at the local box store. Seal one end off, pressurize the other and let sit. You can even get a dedicated gauge to attach for this purpose. The same technique is used to verify plumbing, but I don’t recall max PSI off the top of my head for a closed system.
I would NOT use any on-demand heat source. They’re designed for intermittent use and would not only wear out prematurely, but would likely use a lot more energy than a well-insulated tank-type heater. If a boiler is planned, a small water heater makes an excellent storage tank, or can be brought into service in time of emergency, though probably not critical for a garage slab.
You’re also going to want to use an appropriate antifreeze even if you don’t expect the slab to ever drop below freezing. Radiant antifreeze has additives to help lube your pump and cut down on contaminant buildup. When done correctly, a good closed system should almost never need a top-off, assuming all air is purged.
In selecting a circulation pump, it will be helpful for you to know loop length and diameter (assuming this is just one large loop). There’s some math involved as you need to ensure good heat exchange, run the numbers to correctly size the pump, and attempting to avoid system turbulence.
I'm hoping they used barrier-Pex and not regular, cheaper, potable grade. One is designed for radiant heat and the other for cheapskates.