Hello Mark;
I would also stress the word ventilation, especially in summer!
Most Chinese Cyps. appreciate a rather dry winter - rather easy in a cold greenhouse.
There are quite a few Chinese and Japanese Calanthes, that should be hardy in your climate.
Pleiones are worth a try - a friend of mine has overwintered forrestii outside with some protection here in Austria 2 times, but last winter with -19C was too much. But formosana and bulbocodioides at least should work, and of course limprichtii.
Then there are Bletillas apart from striata
Cymbidium goeringii
If you supply some extra heat for the coldest days, then Disas would be another wide choice!
Perhaps place the house in the shadow of a deciduous tree so that it could not get to hot in summer!
