"Gas fired warm air furnace", now that's interesting. I've never heard
of a cool air furnace, hehehe. Like my neighbor always said, there's
no such thing as a hot water heater, else you wouldn't need the
heater.

Anyhow, you are looking at about a 2 to 2 1/2 ton unit for 1100 sq ft.
Suggestion, find a contractor who carries Carrier, call them out and
have them look at your existing system. They may be able to add the
air conditioning evaporator coil, which is the inside coil, and then
the outside condensor coil, to your existing furnace/fan setup. If the
house never had air conditioning, the duct work may not be sized the
best for air conditioning but may be ok.

I mention Carrier for a few reasons:

1. I have a lot of experience using and maintaining Carrier and other
A/C products
2. Carrier's new line of 'split systems', those which have separate
inside fan and coil and outside coil, have variable speed fan motors
and use a Thermidistat instead of a thermostat. This gives you the
ability to dehumidify during the cooling season even when the
temperature in the house is cool and also other good features. Other
good brands have similar capabilities
3. Stick with a good brand like Carrier, Trane, Rheem

You should be able to find a contractor who can do it all but they
still may sub some of it out if any new duct work is needed.

Are your ducts insulated? You will need insulated ducts because the
condensation will be bad news otherwise. You will have mold everywhere
if you don't insulate. With a house the age of yours and it only ever
centrally heated, there may be some retrofitting of ducts and other
things you don't anticipate. Just make sure to get 3 bids from 3
reputable contractors who carry name brands like mentioned above.
Also, your contract with the contractor should spell out everything to
be done. For instance, if they have to insulate ducts, specify minimum
1 1/2 inch thick insulation installed without compressing the
insulating material, ie, you want the 1 1/2 inches. I got screwed on
my new home because I trusted the contractor after talking to him and
didn't spell out things like the insulation and duct size/shape. The
people who did the duct insulating wrapped it with 2" insulation,
which was a good thickness, but the wrapped it so tight, the
insulation is only about 3/4 " thick. My ducts are in the crawl space
and sweat in the summer because of this. So spell everything out.

Your evaporator, inside coil, needs to be plumbed so the condensate
lines are installed per code. Typically the primary goes into your
plumbing drain system and the secondary/overflow drips outside the
house.

Make sure they insulate the larger suction line completely, from exit
of the evaporator to inlet of the condensor and the insulation needs
to be glued at seams with rubber cement. Pipes need to be adequately
supported.

Make sure you get a 13 seer or greater rating unit, they do pay for
themselves. Look up seer on the internet for more info.

Good luck