Hi Declan,
We generally have the top half of our stable door open as well. Would be good to check if any rain blows in if its particularly wet and windy, but it should be ok. The metal might be a bit cold, but if you deep litter them (give them lots of straw and only muck out every 2-3 months) then it shouldn't be too bad. I might be tempted to put a wood inner lining on, but that might depend how much extra room it would take up, and how much it would cost too!.
Our old property, which we moved away from about 5 years ago, was originally a fever hospital built in 1912. Therefore it was constructed with brick foundations, corrugated iron, and then internally lined with a double skin of wood panelling. Our house itself was the matron and nurses accomodation, but we had a cookhouse and 2 wards as well, and we kept goats in the cook house (and wards sometime) for well over 30 years. The problem with internal wood lining, is that the goats tend to chew it. We didn't have a problem with condensation.
Beth