You'd have to pull the top cover to see what I am talking about, but you don't have to pull the cover to install an air vent. The reason I say about the air vent is because I recently installed an auxillary boiler in my garage and its air vent was at the ceiling, the highest point in the house. Everyday I would hear the air going through the pipes, yet the air vent never bled it out. I finally realized that is was the air in the dome of the boiler and I bled alot of it out through the pressure relief valve that day. Everyday I crack the valve and a tiny bit of air still comes out, but the system is now whisper quiet. As long as you have circulation, your air scoop and air vent will bleed out the air. And any that is trapped in the dome of the boiler can be handled as I explained above.

Yes you need vents. Adding vents depends a lot on what they need to do. Do you have a chase big enough to the top floor for both return and supply? A garage is a good place to build one. Do you have an attic to access the top floor? Then do you have blown insulation up there? I'm going to guess $3 per sq foot. That is a wickedly wild guess and is high for a remodel. I probably paid $1 on new construction (wasn't broken out on overall bid). A chase into garage wouldn't cost more than an extra $1000 I wouldn't think. It might need to be 3x3 feet. The alternative would be heat pump in the attic which would lower the duct costs some, remove the chase costs, and add refrigerant line costs, electrical costs and another unit to install. If you don't have a garage to run a chase and you don't wan't one sticking from an exterior wall, then you have to put the unit in the attic.

Post your electric rate in cents per kw-hr, and your gas rate in either $ per ccf or $ per therm. The heat output of a gas furnace is very similar to electric resistance strips if enough kw are used. Utility costs decide which is cheaper. 5 kw electric strips put out air temps about 15F greater than room temperature. So a typical 10kw strip heat puts out 100 degree air if the room temp is 70F (70 + 30 = 100). This# is based on 1200 cfm air flow from the indoor fan. 15kw = 115F which is more in the range of gas furnace temps. The air handler must be sized to handle this heating load. These temps are for the strip heat only and do not include any btu's from the heat pump. The heat pump itself puts out air temps from about 90F to 100F based on outdoor air temps. The 90F supply temp air from the heat pump combined with 20 kw aux electric strips provides minimum 120F output air when needed. Heat pumps provide enough heat up until the balance point, which is around 32F or so based on home construction/insulation (mine is 25F). At temps lower than the balance point, aux heat is needed - either furnace or strips. The good thing with the strips is they can run along with the heat pump. The furnace cannot run when the heat pump is in heating mode; its one or the other. When the heat pump goes into defrost mode, the air output is in the low 50's. 10 kw provides 30F to temper the air into the low 80's so it doesn't feel too cold while the heat pump is defrosting. The furnace will provide 120F air during a defrost cycle. The heat pump is typically 300% efficient down to 35F ambient. Heat pumps make cheap heat. One comment about the XP-19. It's "up to 19 SEER". Not all combinations get 19 SEER. Model numbers can be used to determine actual SEER. Also, dual-compressor systems like the XP-19 are more for hot and humid summers than for heating. In the cooling mode, running on 1st stage saves $$ and reduces humidity. Once on 2nd stage, it loses its operating $$ benefits. A 14/15 SEER usually provides the best combination of purchase price and operating cost. If you can afford a "19 SEER" system, go for it. Whichever SEER you get, you want as close to 12 EER and 9 HSPF as you can get for low operating costs in cooling and heating respectively. Good luck.

I'm leaning towards just heat pump with heat strips. That should be fine for your area. If you plan on keeping it that warm in the house as you say in your OP, I would get the Bosch Inverter heat pump as it produces heat clear down to about 5-6 degrees ambient and very efficient at what it does. Then use the strip heat as you need it. I don't know that you'll find a contractor though for this for a new construction job as Bosch just recently came to the Houston market a few months ago. The distributor network for Bosch is in it's infancy... so there are pros and cons for choosing this brand. For what it does / how it does it --- it's a game changer for my market. (Katy, Texas area) As a comparison --- an average run of the mill single speed heat pump will probably struggle once temps drop into the 20's for any significant amount of time. They are efficient when compared to strip heat (utility bill cost wise) but there are higher limitations with those. I've gotten email notifications from my electric provider that they are estimating my next electric bill at less than $50 for November. This may wind up being my lowest electric bill ever for my power rate of 12 cents per KWH. With that said, I keep it quite chilly during the winter and my house is all electric (no gas at all). I shut it off at night and really just use it to take the chill out of the house in the morning. If it gets really cold here, I will probably just heat the main part of the house over night --- as I have it attached to a 4 zone duct system. (This is to explain the how and why my electric bill is so low -- design of the HVAC system is a big part of it.)