400' in a single loop is probably too long - better to limit it to around 200' or the water at the end of the loop will have cooled off too much and the head is higher making even heating more problematic. You can drive both loops on a floor with the same thermostat. You should never (well, almost never) size a boiler for your potable hot water needs...adjust the size of the tank. Running a bigger boiler for that purpose means most of the time, the boiler is way oversized which affects efficiency. To avoid hassles with nailing into the pex, I'd seriously consider an engineered hardwood floating floor that snaps together. www.kahrs.com has a huge selection and is top quality stuff. There are others that make similar stuff as well, but Kahrs is really good stuff.