When does an UAV need an internal structure?
I very much doubt that you will hit your mass target (kg is a unit of mass, btw) if you build those wings without a proper spar. The shell is for carrying shear loads to provide torsional stiffness, but it cannot give much bending stiffness, especially if there is no internal structure that keeps the upper shell apart from the lower one under bending loads.

So build a light shell and add spar caps, a web and a couple of ribs where loads are locally introduced or where cutouts (say, for ailerons) start.

Regarding the fuselage: If there are many point loads inside from the payload and systems (camera gimbal, engine mounts) it makes sense to have local reinforcements. Otherwise, a shell should be fine given the higher curvature of a typical fuselage. On the other hand, everywhere you have large radii or flat panels, a local stiffener will be advisable.

As for calculating the shell thickness: At that size I expect that handling loads and inertial loads from point masses will determine the minimum size. There is no simple formula and you need to balance ease of operation with performance. Depending on your priorities, the shell will become thicker for a more rugged device or thinner for a lighter but more delicate one.