Greenhouse plan!

You're going to want sometime with twin or preferably triple wall poly glazing so your heating costs aren't insane. Your options are gas or electric heater. I think gas will be cheaper to run. Southern is a popular brand. I would also get a bigger size now if you can afford it. A bigger one will not experience as large of temp swings and you'll just wish you had gone bigger in a year or twoz

 

Personally I would just buy one but building is an option if you're handy. You can certainly get triple wall greenhouses as kits. I think you need to up your budget a bit. I think the ones you are looking at are more for extending the growing season and starting seedling early for regular old gardeners, not growing orchids year round

 

I have the 6x8 Palram model that you posted, and it is twin wall. I put mine up at the end of winter this year, so it didn't have to work all that hard to keep temps up. It did keep temps above 60F when it was ~40F ambient outdoors with the help of just a small space heater, so I think it can keep me at least above 50F when we freeze. Take my advice with a grain of salt, though, our climate is basically sub-tropical. If we see a night at 28F, it's treated as a major event. Heaven forbid it rain and be that cold...

 

Jai,

Go to this heating calculator, and it will tell you the approximate size of heater needed:

http://firstrays.com/free-information/growing-conditions/sizing-a-greenhouse-heater/

 

I've looked into these pre-fab kits in past, but speaking as a fellow frigid climate dweller (Rochester), one potential problem I see is entering and exiting in the winter. Unless you have a vestibule (like a double-door airlock system), all your heat is going to go straight out the door the minute you open it. And the cold blast on the plants can't be good.
- Stephen

 

You'll want shade cloth, which ought to be enough unless you focus on cool growers.

 

If you are going with a green house, have you thought of wrapping it with bubble wrap in the winter. I wrap mine on the outside and it does keep it warmer.