Insulated PEX considerations for small outbuilding

Trying to plan ahead as we need to dig the trench soon.

In a couple years we'd like to install a Garn at/in the house.
Now we have a 12'x24' outbuilding that looks a lot like a "guest house" located about 100' from the main house. In the next few weeks we'll be running electric, gas, water, and a spare conduit to the outbuilding.

Seems while we're digging, laying, and covering up lines that now would be the time to run some insulated PEX for future heating and hot water use in the outbuilding.

I've read on here a lot about foaming in the trench and that sounds attractive. However I wonder if considering the intermittent use of the building and likely smaller size pipe needed? if a pre-packaged solution may be comparable and or easier, perhaps cheaper in this case.

The building has 4" stress skin panels with white styrofoam in them. Again it is 12' x 24' and one story with about 12/12 pitch roof. Does have a large 9' wide double pane glass window/door/window on one wall. To heat we'll likely use a radiator or perhaps staple up PEX radiant. I wonder if the loop could also include a plate and plate heat exchanger to provide hot water for a shower?

Questions:
1- Can we heat and have hot water with only two insulated lines plus a cold water line in the ground
2- What size insulated lines might we need?
3- Considering the above what sort of insulated pipe would be suitable.

Options as I see them, in no order:
Spray foam in trench
ThermoPEX
MicroFlex (page 53)
Logstor
INSULPEX

Thanks,
It doesn't look like any of the brands I was looking at offer anything less than 1". Haven't done any calculations yet but from reading here seems like folks are heating a lot more space and using 1", so likely that would also work for us.

You're probably right though... something like a vented gas wall heater (the original plan) will be a lot more economical up front and continue to be for a long time compared to the insulated PEX cost.
I do already have a gas water heater for the building but still need to make a small bump out on one wall to have a good place for it to live. Using hot water from a wood boiler for both heat and DHW seemed like a smaller and simpler setup in the little house. But you're right, more expensive, likely more than the use justifies.

Thanks for the thoughts,

The porch deck on the left didn't really need to be timber framed, that's why it looks different.

Here are more pictures and a little info on the timber frame, top of the page is most recent, it gets less and less done as you scroll down.
https://flanamullegan.wordpress.com/