Shed Insulation

Hi I've recently built a cheap 10ftX10ft shed and want to insulate it.

Since it was cheap it has overlap cladding and only 25mm deep posts.

What would be the best way to insulate it?

At the moment I'm either thinking
12mm celotex air gap on the outside, then the same polythene DPM on the interior side then interior cladding with 6/9mm ply.
OR
25mm celotex (no air gap) with a polythene DPM on the interior side then interior cladding with 6/9mm ply.

The roof will have 50mm celotex, with DMP and maybe ply cladding.
the floor will have 50mm celotex/polystyrene with some sort of boarding over the top and a carpet

I'm also thinking an electric heater/radiator will help.

Is 12mm celotex going to be worth it?
If not, how needed is an air gap between the outside and the celotex?
Am I going to be better off adding more thickness to the posts to create an airgap?

Am I even going in the right direction with this? is there a better way?

Thanks for any advice,
Nathan