New heating unit & hot water heater for small apartment

[Nadene Grey]

Hi,

I'm separating the gas on a small 2 family building. I will use the existing boiler & hot water heater for the first floor apartment. I'm trying to find the most economical cost for heating & providing hot water for the second floor unit which is 588sq ft, has R30 in the attic which is the ceiling of the apartment, no wall insulation, has 3 external walls, 8 single pane windows with storms, and the floor will most likely be heated from the apartment below. The closet I have to install the furnace/boiler/combi/whatever you suggest is a 4' x 2' closet & vents can go out the wall or up the through the roof. Please advise.