The window evap cooler units are not as efficient as a wet wall on one end and exhaust fans on the other end. Mine has the later and I can keep it as cool or cooler inside than out. Today was an extreme example 100F outside which gave about 68F out of the wet wall and a high of 85F on the exhaust, hot end. With much higher dew points in summer it's still as cool inside as out with no shade cloth. The window evap units are inefficient because they mix the cool air with the hot inside and a mixture of that leaves the house. With exhaust fans pulling the cool in, the hottest driest air is pulled out. I think an efficient evaporative system would still be a huge help in MN. Much better than anything mentioned above. A shade cloth, if you can stand lower light levels, is an additional considerable help. I'm growing all kinds of fruit in a greenhouse in SW Texas without shade cloth. The crops do very well and fruit quality is very high. But most of these crops are OK at 90F high temperatures. Greenhouses with efficient evap cooler systems are used to grow tomatoes most of the summer in central TX where it is hotter and more humid than here, upper 90s highs and 70F dew points. You should be able to grow almost anything in summer in MN.

tete-a-tete, That is basically what we have done. When we built, we used blown-in insulation, as our landscaping matures, it is beginning to help cool the house. We use window treatments and the opening and closing of windows at the right times to help regulate the temperatures and use ceiling fans. For us, part of it is budget. We could not own the home we do and spend a lot of money on climate control. Part of it is environmental responsibility. One thing that helps us though, is our climate is generally dry and the temperatures drop significantly in the evenings. However, tight insulation restricts air flow so when the nights get cooler, there is a period of time when it is much more comfortable outside. If I were to build again, I would have added a sleeping porch upstairs that was screened in for some of the summer nights. Some things that help too are that I work at home, so I can manage the opening and closing of windows and the windows can be open during the day as someone is home. If I was not home, the windows would have to be closed as a theft deterrent and in case of rain (which would be bad for the wood floors). Also, all of our sleeping quarters are upstairs so we can leave the windows open with less worry. I would not be inclined to allow my kids to sleep with open windows if they were on the ground floor. It is a complicated thing.

Although New York City may be and expensive place to live and install central air, most of the State is not any more expensive than many other parts of the country. Check into the NYSERDA program. The need for cooling varies a lot between the humid and relatively warm costal plain and inland mountains in the North. Lots of choices to be made and if you make them before contacting contractors, you can get a better comparison. If the OP could be happy with the cooling performance of good quality window units, then mini splits could work quite well. I really don’t like the idea of outdoor ducts, whether they be in the vented attic or crawlspace. In a home that size, you might be able to get away with a minimally-ducted mini split and a wall unit or two, or two of the former. The minimally ducted units can be installed by sacrificing the upper reaches of a closet or two, or in the upper space of a basement stairway. One AHU can supply directly at least two rooms that way and often three or four. IMHO, mini splits are a made to fit solution to adding cooking to a fine hydronic heating system. How is the domestic hot water supplied right now? Assuming that you don’t have steam, if you decide to stick with hydronic heat, please get someone that knows what they are doing or do your own research. Plugging a modern, low mass modulating boiler into an old system can be tricky. If it is not done right, you will get less than optimal comfort, no payback from a more expensive boiler that could have saved you money, and a broken down boiler before its time. If you have all the original radiators and have (or will) improved air sealing and insulation in the house, you are a long way towards a situation that can take advantage of an efficient boiler. That is because you can get sufficient heat out of excess radiation, especially cast iron radiation, at relatively low water temperature. In a home that size, I’d consider installing a Westinghouse WGR050NG076 water heater as your boiler. They modulate 3:1. They are efficient. They are inexpensive; the appliance can be had for about 2 kilobucks. You don’t have to be rocket scientist to install them in place of a high mass boiler because they are high mass boilers. Little or no modification of your current piping would be needed.

I have no experience with this either - but I do have experience with gas furnaces. I can't imagine a conversion that would be the right answer. New gas furnaces are easily 92-95% with condensing secondary heat exchangers, outside air intake, venting through PVC and ECM blower motors. All of those things are great to have. Venting through PVC solves venting issues (lining a masonry chimney, for example). Outside air intake reduces air infiltration. ECM blower motors ramp up slowly and use less electricity both in the winter and in the summer w/AC. You will give up a lot trying to do a conversion - and I can't imagine you would save much money.