For a 600 loft in NY I'd think a cheap 110V 3/4 ton mini-split would be a more appropriate choice for heating since it cools too!
They all draw less than 10amps, and put out more than 10,000BTU/hr at +15F (NYC's 99% outside design temp.) If that's not enough, the 1- ton versions draw about 10A.
The gas plumbing for a monster burner like the Navien is critical and you may need to upgrade the regulator if you have other big burners on your gas system. At 17,000BTU at 130F average water temp it takes 60' of baseboard to balance, and at 120F AWT it would take about 75'. If you go with much less than that you'd have to crank the temps well above the condensing range to keep it from cycling the burner on/off.
Start by calculating the heat load on the place, THEN you'll have a better shot of figuring out what it takes to heat it. If it has insulation in the walls/ceiling/floor and glass in the windows it probably won't be anywhere near 17,000BTU/hr anyway.