A few things others noted, the BBB is a joke for you to put any faith
into. Your prices quoted are sounding very high for what you are being
offered. I don`t know your total heating bill for a year but you say 550
for a month of feb. Figure how much your actual yearly heating bill was.
There are high efficiency condensing furnaces which are the norm here
and do save alot more than an 80% unit. Different manufacturers have
different efficiency ratings , they go from apx 93% to a new unit that
has 97%. The savings difference is figured on the differential of the 2
units. A 97% will save you 21% over an 80%, not 17% as many think . I
personaly would not consider an 80% unit unless yearly heating was only
a few hundred. You say Feb was 550 so im guessing 400 was heat so you
qualify for high efficiency condensing unit, and that was at last years
Ng prices, not todays or in 5 years, my prices are up 70% this year.

You state no ducts can be run to 2 rooms, I find that real hard to
believe that something can`t be done as I have just upgraded a 110 yr
home without heat in 3 rooms. Flex duct for "Spacepack AC" is run
everyday to retrofit old homes and can be enginered to do heat, Ive done
it for a hard to plumb room. With a house of that size you are
forgetting VS DC zoneable systems that will save 25-50% in running the
blower. They are available on the best condensing units and can remove
50% more humidity in summer and give more even winter heat through
slower fan operation.

For 2900 Im getting a Carrier infinity 94% with VSDC and special
thermostat instaled, your price of 6300 for 80% units sounds extremely
excessive.

You had a few unqualified bids that did no load calculation as I see it
ranging from 100k Btu to 200k Btu. 100k btu 80% efficient gives out 80k
Btu heat. 200k outputs 160k Btu. At 96% efficiency 100k outputs 96k Btu
and 200k outputs 192k btu. A 20% increase meaning a 20% smaller unit is
needed. A great difference in output you should consider in sizing is
output and their efficiency.

For probably under 4000$ you could get a 97% VSDC out putting 120000
btu that is equivilant to a 80% 144000 input unit and save winter and
summer through its VS DC motor. With some insulation even this unit
would be oversized for your area.

You need to learn about what you need and what is offered and run some
real numbers on utility, equipment, costs and paybacks. Run the numbers
with todays higher Ng costs and double it for 10 years, then you will
realy see that an 80%er is dumb.

You only need the unit sized to keep you warm on the coldest days
running 24hrs a day, If you are going to keep the place future
insulating will make the difference. Caulking everything will reduce
wind iniltration. Insulating curtains and Cellular shades can take the R
1 windows you have to R 5- 6+ likely that would reduce your Btu needed
by a very great amount. If you can do it figure the load calc several
ways for true sizing needed. I sized and had my AC instaled before I
knew this or insulated, so I regret im oversized and humid. There are
many sites on heating, windows, insulation you need to learn as your
"heating pros" are not helping you they are just instalers. Im no pro
but in zone 5 1800 sq I paid 465 last year for Ng down from 2000 by
doing my own research and following my ideas, not any salesmans.