Gas and Oil Home Heating Furnaces - Best heat source for attic conversion??

Working on an attic remod in my home and am trying to get the hvac figured out. The original upstairs construction had 2 BR with only passive heat, (old farmhouse). I am adding about 350sf of finished area to bring the total living area up to about 1000sf. There will only be 1BR now with the rest of the area set up for a loft. (Some day it'll be a rental)


The house has gas already, so I am leaning towards that. From my experience, electric heat is too costly.


What I am considering now is adding a wall mounted gas forced air and venting it through the roof. I have about 64sf of dead space surronding the existing chimney. I was plannning on using this area for the furnance. I don't plan on running any duct simply because it is not feasable, so the furnace would heat directly into the loft area. I am planning on getting a 1500w baseboard heater for supplemental heating in the BR. The BR has an open stairwell in it so it will receive some passive heat from the forced air unit.


Now the Q's that I have come up with....

How much venting do I need in the wall for return air??

Since it is venting directly to the roof and the run is only about 4', is there any special sort of backflow preventer that I need to install. It would be on the SW side of the roof, which is the prevailing wind direction here in MI.

Any recomendations on MFG?? Something with a fairly quiet fan would be nice. We had a wall mount LP in our cabin and that thing sounded like a plane taking off every time it ran!


Thanks for your help!!