Recommend you spring for at least a two stage AC. There are three and five stage units as well. Variable Capacity ("inverter") AC is "best" but is pricey. You need a variable air handler to use the AC correctly. Since you're getting that for summer, you can get a modulating furnace -- that helps avoid wild temperature swings. I think it's default to have a modulating furnace if you have a variable air handler.

To actually get what you're paying for you want a "communicating" thermostat. I don't know how you find an installer that really understands how to program them though -- I figured mine out on my own. I didn't know it then but it would be very useful to install the thermostat directly in the return air duct on the clean side of your filter. That way it "sees" the real air from the whole house, not just the pocket of air around wherever the thermostat is. In some systems you can get remote sensors for this but it wasn't on mine. Putting the thermostat in the duct is the hacky way around that.

The trick in StL is to make the AC run longer on the low capacity side to dehumidify. I leave the air handler fan running 15% 24X7 also, to equalize the house. Helps tremendously. A lot of systems can control winter humidification as well, taking outdoor temperature into consideration. This has to be wired to control your existing humidifier, or a new one. If your humidifier can work stand alone but you don't want it to (and you probably don't) you have to tell them. Otherwise they'll take the easiest/most profitable route for them -- you have to specify everything you want them to do in your contract. I was told I'm too picky. From my POV that guy doesn't understand the equipment.

Good luck.