Thanks for all the tips. I'm going to have to do some "in depth" research.
18" in the case of a/c power
Yes, use conduit for both mechanical protection of the cable and the ability to easily pull new cable should you ever need to. And definitely use warning tape above the conduit, and install a tracer wire to the conduit.
If you have a property survey, attach a note with a sketch showing where the buried services are.
