1. When you use chilled water for air conditioning, you have to have fan coils...blower units. Heat works by convection but cool doesn't work that way. Each fan coil needs power, condensate drain, etc.
2. Ductless might be a good choice for the A/C. Many brands now make what the call inverter units, meaning they can be multiple zoned. One outdoor condensing unit can support up to three, possibly 4 indoor units. An air handle in every room might be overkill, and would start to get pricey. GOogle "Mr. Slim" which is the Mitsubishi line.
3. If you have any gas burning appliances, especially larger ones like heating and water heater, then tight houses must make provisions for an outside source of combustion air. Also, tight houses can have issues with "stale" air. Fresh air can be brought in through an air-to-air heat exchanger.
You need a really good HVAC company to evaluate your needs. Winging this stuff is iffy.
4. 1½ tons sounds small for 2100 sq. ft, but you are well insulated, and I don't know your climate, which is why you do the load calculation. Having said that, I was reading a study recently that showed most contractors tend to install 1/2 ton bigger, because homeowners will complain about the performance of a system built right at manual J. The system will be very efficient, operating in long "on" periods, and doing the best job of dehumidification. But if you come home on a hot day, and the a/c has been off, the homeowner wants to turn it on a feel a blast of cold air, from a bigger system!