What does the instruction manual say? It is considered a furnace which requires a separate circuit. Some locations require these furnaces to be hard wired, while others permit the use of a line cord plugged into a receptacle. It provides the disconnect close to the device, otherwise a switch, etc would be needed if hard wired.
A 15AMP circuits using 14/2/w/ground is fine.
Sounds like you are doing a good job of planning. I'm sure you are giving thought to installing the thermostat wire. My only regret with the furnace project was not providing a conduit for it instead of burying it in the ceiling.