Jeff, add that upon any news of frozen water in the forecast, the majority of the population here rushes to the stores picking the aisles clean, stocking a month's worth of perishables that will go bad in a week! Other than that, you nailed it!
BBP, you are absolutely right on preventing short cycling. I'm sure yours works well. The simplest way to do it though is to use an Energy Star conventional tank water heater (most are certified for hydronic/DHW combinations) and an open system (safe and approved in most locales). A 40Kbtu burner on a 40 gal 12 year heater for $600 is plenty because of the "buffer factor" of the tank AND floor/house unless your house and household is huge. I'm assuming anyone would already have a decent building envelope and water efficient fixtures such as Water Sense.
The water heater will have less work to do, since it will never see inlet water temps below your interior temp. The incoming water replacing the DHW being used is drawn through the floor first. More simplicity, less cost. You would have to have a LOT of DHW use combined with a peak heating time before you would overwhelm the burner and heat already in the plumbing and house. Energy Star's have a 67% or greater energy factor, which is probably about an 80% efficient burner.
Tankless, tankless hybrid and boiler systems have trouble if they can't throttle down to the lowest demand level (say, a faucet trickling warm water, or a single 0.5gpm heating zone). Conventional tankless has an awful lot of head loss in it's coil.
My Eternal water heater is a condensing "hybrid" with low head loss and a few gallons onboard storage but it still can't throttle lower than 26kbtu. It's 96% efficient unless short cycling or when warm loop water is returning. At 110-115F return temps, it does still condense some, but I imagine the efficiency is more like 85% then. The newer, smaller Eternal can throttle down to 14kbtu, but I'm not sure even that is low enough, and I would replace the plastic pressure sensor with a metal one before I would even consider an Eternal again.
A condensing boiler won't condense much as you have it installed since it is circulating already warm water, but condensing and noncondensing boilers will still work fine. It will operate most of the time with an efficiency not much better than a conventional Energy Star tank style though.
Of course, I'm advocating and switching to a solar/geothermal hybrid system to get heat for "free".