Indoor garden room

[Kombi Camper]

Looking for some general construction advice please ?

I am about to start building a timber 'garden room inside an agricultural building. The agri building is not insulated, it's quite rudimentary and will mostly just provide shelter for the new timber building from the wind and rain. 

The 'garden room' will be used as a clean workroom for mum (painting, hobby crafts etc) and an office space for me (desk, computer, photo printer etc) so it needs to be dry and warm. Dimensions approx 16x12ft or 5x3.5m. I already have by chance brand new upvc window and door for it off a friend. 

I've watched a few youtube vids and am happy I know how to make and insulate the base. I'd like to keep my costs down and wonder if I need to make the building so strong as the roof will just be shed felt (the thicker stuff not the paper stuff). Is it overkill to proceed with 4x2's (CLS equivalent) and 100mm PIR insulation etc. Afterall, this is a building that is essentially already inside and just needs to be warm and dry. Would you use 3x2 (metric equvl) and 50mm PIR for walls and ceiling? My reason for using PIR is it's great U value in shallow thicknesses and the fact that it will be a vapour barrier if I tape all the joints and timber faces well. Any other ideas to save on costs?