Welcome Jade!
I live in Colorado Springs CO at an elevation of 7,000 feet. My orchids do fine. Have a few friends who live up in Woodland Park CO at about 9,000-10,000 feet. They both have greenhouses that are bursting with glorious orchids. One of these friends is an AOS judge. She wouldn't be way up there if her orchids didn't survive and thrive.
So, GO FOR IT!
I don't have a greenhouse, but rather an atrium. I have closed off the entry ways with clear shower curtains and run 2 small humidifers (bought at Walmart) to maintain 60% humidity. Used to have a bigger unit but the fan broke. Haven't noticed any effect on the levels.
Seedlings, in my experience, are far more delicate than mature plants. If it were me, I'd start with mature, or near mature plants.
And that's my two petals worth. What do others think???