PEX tubing, How deep?

Definitely use antifreeze glycol mix.

What you might consider doing is running the supply and return line as a larger single pipe to a point inside the dwelling where it goes into a header manifold then distributes/returns to each zone.

This way you only have one supply and one return pipe to deal with undergound and/if they ever froze or leaked, then it would be a simple repair than dealing with several pipes.

Another item would be a single wire on the outside out the plastic pipe. This could be used to thaw out a frozen line. I sure would go atleast the depth of my frost line for your area.

I would also look at putting closed cell pipe insulation around the underground pipe then slip another protective pipe over that. This would keep the "underground critters" from tearing up the insulation over time.

When you double the pipe size, it will carry four times the volume. So a 1-1/2" pipe will carry the volume of four 3/4" pipes.

As the water temperature rises the water will expand by approx 20%, so be sure to include a expansion tank in your plans at the high point of the system with a automatic air bleeder.

T_Bone