Oh so give all your money to the window guy and let us hvac guys starve!
I'm just kiddin' Your windows will benefit you in the heating season and is the biggest heat loss/gain you may have in your home.

Back to you,
your best bet will probably be mini split ductless systems.
You can also get them 3 zones.

They are not the price of a window unit but less than a complete system with duct.

it never hurts to get quotes, once you get some quotes you then can determine if the cost justifies being cool 2-3 weeks out of the year.

We really didn't WANT to drop a bundle for all new windows, but 95% of our windows have seal failure, and we can't even see outdoors anymore, so the purchase is a must, unfortunately. For finding a HVAC company, since we don't know of any through word-of-mouth recommendations, do I choose any, or should I ask if they have experience in all three types of AC prior to setting up quote times? We looked into getting central air installed about 5 yrs ago, and out of the six names I called, only two called back for quotes, and only one ever came out to give any estimate. I hope we have better luck this time! Any reason to automatically eliminate the high-velocity mini-duct system? Thanks again for any and all help!

My cousin lives in a large 2 story that is mostly cooled upstairs only. Conventional ducting in the attic, supply air to each upstairs room and 2 poorly located in the main floor as her Mom wouldn't let them be put downstairs where they would be seen. Even with that, downstairs stays comfy just from cool air falling and warm air rising.

I wouldn't use high velocity unless you are trying to snake ducts to the first floor. Just use conventional ducts. Only reason to use mini-splits is if you can't do a system in the attic above the 2nd floor.

Thanks for your help. Again, due to financial reasons, if we can get away with doing the second floor only, we then can save money to have the first floor done in the next year or two. I'm hoping whatever we pick out will work fine. I don't see alot of support on this site for the high-velocity systems. Our attic is unused/not finished, so we would not miss the space used there for conventional AC ducts.

Ducts outside of the living space (attic) are a mistake which makes minisplits the probable better choice. There are a lot of efficiency losses associated with ducts in the outdoors.

You can install as many as many as 8 indoor units for one outdoor unit, though it may more economical to install a couple of smaller outdoor units. Consider heat-pump minisplits. Depending on fuel/electricity costs and the performance of the equipment, you might save money with the heat-pump function, but you probably won't be able to use that when it gets really cold.

The big problem might be to find installers familiar enough to get you the best systems. There are more options all the time in addition to the "traditional" wall-hung indoor units.

To get to the heart of your question. The money. Here are some "ballpark" figures. Please verify your local rates and factor in the age and condition of your home and its electrical system.

Buying and having installed 3 ductless minisplits, along with electrical and structural work is going to be a ballpark estimate $4500-7000 depending on how much electrical is needed, and local labor rates. I agree with previous poster to get these as heatpumps if this is what you select.

As these units are frequently used in commercial applications for computer and elevator equipment rooms, I suggest looking at an HVAC dealer that does both residential and commercial.

Compare the cost/benefits of these systems vs going an airhandler in the attic and adding conventional ducted ac. (Which I have personally in my 1931 home with boiler heat, and highly recommend. ) A conventional system will have better warranty and will have a longer expected system life than minisplits. So this factors into the cost side of the deal. Depending on local labor and electrical, you may not pay much more to add a small heatpump system with ducting to your home. Advise looking at a heatpump if you do this, as you can run the heatpump for some nice heat spring/ fall when the days are warm and you don't need the boiler on, but nights get colder. We have this and it is very nice.

Also agree with the poster who said that improving insulation is worth the money. It will likely be around $1000 or less to get insulation blown into the attic. Your home will just "feel" more comfortable.

Have you considered portable ac units? We have LG brand works great we only wanted it for our second floor bedroom.

The downstairs stays cool but the upstairs gets really hot