Adding Air Conditioning

We built a house 18 years ago. At the time to cut costs, air conditioning was something we did without. I am to the point that I am tired of sweating my ass off in the summer. So my options are: 

1) add ac to the existing furnace.

2) put in a new furnace with ac.

3)?

My biggest dilemma is that we are only going to be in the house 2-3 more years. I'm sure a new furnace would be an upgrade, and probably not a lot more than just doing the ac. However, I also don't want to spend a ton on something for such a short time.

How big a house? How much does not having AC hurt the resale? I'm assuming not much, if it was even an option when building.

You can throw in some window units for pretty cheap. You can also try a minisplit unit, which may be the best option from a cost standpoint--you'd probably recoup the cost in resale.

Adding A/C shouldn't be difficult or expensive to an existing forced hot air furnace. [edit] If you have hot water heat, central air is probably not feasible on a budget or for short-term occupation.

They will need to refabricate the basic plenum to add a coil for the A/C, in the main return just upstream of the furnace itself.  That will be piped via copper flexible tubing to a condensing unit outside.  No other real changes should be necessary.  You will probably need a new branch circuit to power the condensor, which they should be able to install.

I would think you can get it done somewhere in the $4k-$5k range for A/C only.  I would bet anything that you will more than make that back in resale value.  Not having central A/C in a house is just like not having it in a car, unless you live in the far north.  You couldn't pay me enough to go back to window whackers.

At 18 years old a gas- or oil-fired furnace is well into its lifespan, but not done yet from an operational standpoint.  Although it will not be at the current standard of efficiency, it's new enough to be way better than the old 60% units common up through the '80s.  Replacing the furnace will add another $5k to the project, but you may or may not get all of that back in resale value.

I have a mini-split and it works fine for our needs. House is heated with hot water, so no ducts.

The house is 2 story, 2250 square feet, with gas forced air furnace. We have a couple window units now but they only really cool the rooms they are in and I don't really like the idea of having one hanging out of every window. If I was staying in the house long term I would probably start over and replace the furnace too with a new high effiency unit. I think the right idea is to just add ac to the existing one. What kind of costs should I expect to do this?

The easiest and best way is to get a couple HVAC contractors in to price it out for you, but as I said above, it will likely be in the $4k-$5k range.  At least in my area on the East Coast.

I cool my old house with two window units, one upstairs one down quite well.     Ceiling fans also are a big help.

Until it gets hot/humid I cool the house with a window exhaust fan and open windows when the sun goes down.