Best value materials for plant room re-build

Hi Guys

Rather than demolish our old log store. I’m considering re building it as a plant room and storage room.

The wooden sides are rotting as are the internal supports. The roof is peeling too and suffers from condensation.

Thinking about a new frame with composite cladding with wall insulation and an insulated roof.

We have some upvc windows and doors being removed from our house so can re use them too.

Any suggestions for a material spec for our builder to quote from would be appreciated.

Thanks