if you're gonna do a boiler, starting with wood fired and switching over to geo is easy. its just a different way of heating the water thats flowing through your floor. you could even leave the wood system in place as a backup. however in a power outage event, you wouldnt be able to circulate your water without a secondary power supply in either event.

geothermal's are expensive to install, and just like your septic field will likely require a planned out space that wont be regularily driven over with heavy stuff if you're going to do an inground loop (very common configuration). the piping is usually at least 48" under the surface, but if you can avoid it you still dont want it run under like a driveway or some such.

my folks have had a water loop for around 20 years now, but ours in on a forced air system, not in-floor. we did retrofit it into an existing unit in place of our then jacketed wood stove. those <$20 ac bills in the summer make my dad smile and my mom put an extra blanket on the bed lol

make sure you insulate the [beeep] out of your flooring before you pour it, or you'll lose all your heat into the ground. they make a cool stapler thingy for anchoring the pex down to the foam sheeting so you dont have to bend over.

you put foam down, install the heating pipe on top of the foam and then pour your slab over the whole thing..

with a system like that in play however - you loose one of the biggest benefits of geothermal - the cheap air conditioning in the summer. many geo systems that use non forced air heat are paired up with mini-splints to cover that part of the climate control. i'm not trying to talk you out of in-floor - its awesome stuff, just something to consider.

if you could manage to find property with a consistent flowing water source you could setup some type of hydro generator and possibly end up mostly self sufficient.
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