The best boilers made for radiant heat are "condensing boilers" because they are very efficient at low temperatures. If you put in enough tubing (4"-6"spacing), you can heat the dwelling with 85F-90F degree water. This would also make it easy to change over to solar heating with condensing boiler back up.
Here are some brands: Buderus, Viessman, Knight, Munchkin, Slant Fin Condensing. All of these brands are 93%= efficient at 90F degrees. This sounds crazy but try to get 4500-5000 ft. of 1/2" heatpex tubing in the floor. My favorite is the Buderus GB142/30, a very efficient condensing boiler with a lifetime warranty.I have installed about 25 of these and have had NO problems with them. As long as you use a condensing boiler, lots of tubing and have a well built house, you should be fine. All models come in propane or natural gas. Good luck! Contact me for questions or comments! firstname.lastname@example.org Also consider 1 zone with constant circulation! This will be the most efficient. I can show you an easy design that works really well. Good Luck!.........................
Radiant heat and plumbing and solar, northern New Mexico 30 years.