You should still have a fan coil system to provide air exchange in your house with boiler heating. If you do have a fan coil, you can install an AC coil in the plenum just like in a regular household furnace. Probably in the $7-12k range depending on the size of your house. You will need a condenser unit outside in your yard somewhere with power run to it.
If you don't have a fan coil setup,the most cost effective solution is to install one. You don't want to head down the road of installing a chilled water system for cooling. Way more expensive and high maintenance.