floor in lean-to greenhouse

[apm]

Hello,

I have a Elite lean-to greenhouse at the back of the house accessd through the kitchen door. We use it as an extra room mainly for storage but it can be nice in summer.
The reason we did not go for a proper conservatory was cost and the fact that the kitchen sink drain comes from the kitchen wall into this area, and there is a large vent pipe running up the housewall, which would have had to built around somehow.
It is built onto level paving slabs which was orginally a patio.
I sealed it all round the base, and got some small tanellised pallets from a garden centre, which I fitted into the floor area, and the on top I put some coir matting ( like door mat stuff) simply because we have never thought of it as a living area.
I would like to improve it a bit by fixing some plywood sheets onto the pallets, then carpet tiles onto the plywood.
Should I first lay something under the tanellised pallets onto the paving slabs for dmap proofing, and would I be best using marine ply?

Thanks