Which brand should I get? Radiant heat

I have installed radiant heat in an existing building, and am expanding into two attached buildings. My radiant heat guy suggested a Navien as opposed to a tank installation. After reading previous posts here, I am not sure what to do. My situation is somewhat unique. I have an ambitious project and little money. I have been building for over 11 years and am just ready to put in heat in the two additional buildings

Usage:
Radiant Heat: We will have radiant heat in three buildings: 8,250 sq. ft altogether.
Domestic Hot Water: We use a photography darkroom where the water could run at 70 degrees for 10 hours
Domestic Hot Water: just two people. normal usage. washing machine, showers, kitchen usage.

I am open to suggestions. many thanks.

Michael

Without a design-condition heat load number and the water temps required for the radiant at that load there is literally no answer to this question.

But we know for sure and tank-less water heater is not the answer.

Polaris. Handles about anything you throw at it.