Here is what I would do if I was you. You don't need that many btu's to heat up them two rooms. Plus you want to get hot water also. haved a certified takagi tankless water rep. come to your home. ( this will be free ) tell him you want a tk-3 pro. ( better heat exchange in the unit itself) it can fire up to 199,000 btu's and down to 11,000 btus. then you want to use a bronze bump with a heat exchanger on one side, plus boiler trim. (1/6 horse power. with a mixing valve for your domestic water. you want to then put two zones on other sided of a heat exchanger with a zone board for 2 zones with 2 pumps one for the baseboard. Ask for high trim baseboard so you can set the water temp down and get max btu's per lininer foot. set the temp for 150 degress durring the winter cold month. Now for the radiant floor get a viega dealer in ur neighborhood. get climate panels that you can put cement nails in and the put a flooring on top. If you wanted to know more about this rout just leave a post. you can have all 3 comming off one unit. The best thing about this type of set up is that you dont need 180 degree water like a boiler. and in summer time you can set the water temp down to 120 or 115. and when your away on vacation you use no gas. The down side is hat the climate panels can be expensive, the tankless water heater has to have a nice size gas line. the equipment to do this job can get pricy. I mean really pricey. with nice equipment comes a good price. Takagi & viega are my brand of choice. but you could use navian and wirlsboro if you wish. all 4 manufactors will be happy to come to your home and sit dowwn and talk to you. ( I sell all four)

You failed to mention openings, windows, ceiling height, insulation, floor insulation, perimeter insulation, additional infiltration, air changes and geographic area garage is located. But based on the tubing, and building size, you could need/use and much as 50,000 Btu's net, which exceeds a standard electric water heater with a 4500 watt element(s) by 35,000 BTU's Water heaters make lousy boilers besides.

"Also have mixing valves at each zone with out door reset. So you can control the temp of the water by the outside temp." This. The boiler needs to be sized for the design limits, and the rest of the time (actually almost all the time) is oversized. Using a boiler wit an indirect tank and a reset thermostat allows the tank to serve as the buffer between system size for the coldest hours and the actual needs of the house when temperatures are milder. The boiler heats the tank and runs hot enough to not be damaged. The temperature in the tank is controlled by a reset thermostat based on the outdoor temperature. The zone valve allow final tempering for each zone based on their actual requirements.

Do you have propane fired ANYTHING? That would be the killer for me. Almost 100% of homes have electricity. Very few homes have propane hook-ups. Some have them for their outdoor BBQs or their outdoor KITCHENS (which are tens of thousands of dollars worth of outdoor renovations). And hydronic (water/gel/oil) tubing can be driven by it's own boiler (like a hot water tank but only for the flooring). It can be fired/powered by electricity or by natural gas, or propane. Your choice. If you find it difficult to access propane or too expensive to go with the tubing you might as well go electric. And to be clear, the tubing is more expensive and takes up more floor space and requires more work (labour costs) and COULD LEAD to a FLOOR HEIGHT raise that is not very comfortable to deal with. But I'm making assumptions without knowing how your kitchen subfloor (and ceiling in the basement) is set up.