Hi Veg,
I know someone who converted his shed to a greenhouse
What you can do is take out the boarding ( needn't be to the base just to the level of your staging) and then I'd replace it with either Polycarb sheeting ( the clear stuff ) or Horticultural glass - it wouldnt be hard to fit it.
The roof I'd replace with the twinwall plastic sheeting it's a better insulator than single and the fact that it's slightly Opaque wont hurt. You can leave the guttering on and perhaps even think about making a false back for it peinted white or covered with a replactive folm ( like Mylar) it will help to stop the plants being drawn