I just don't think it is worth going radiant for 200sq ft. For straight heat I'd use a Rinnai ex08. For H&C I'd go with a 9kbtu minisplit heat pump. She will be very comfortable with either. That is unless you all ready have the tubing in the slab. Then I would use a Rinnai E50 C or a Lochinvar Cadet. You can heat that slab with a teapot. Both the takagi and Navien's are big btu on the high end. I believe that you will rarely get off low fire on this system. Both of my Boiler, not water heater, suggestions will fire from 13-50 kbtu and both come with the outdoor reset program built in.