How deep do you want to get into it?
You could line the roof and floor rafters with foil backed OSB for reflectivity and you'd have the basic walls and floor of your grow space. If no roof insulation was there this would be the time to install some R-38 fiberglass insulation to keep outside weather from influencing your indoor environment you being in a colder climate. 600 sq/ft of space. OSB is 4 x8 = 32 sq/ft. 600/32 = 18.75 sheets of OSB but make it 24 for cuts and things. HD website says foil backed OSB is $12.60 a sheet. 24 x $12.60 = $302.40 USD.
Not sure how you'd be able to use anything under 3' high so I'd probably mark that as my wall spot so I'm not having to air condition wasted space.
Are you going to run a sealed room or a ventilated room?
If your attic temp doesn't change that much through the seasons an easy way to line your attic would be 1" R-Tech foam panel insulation. This would only work for the roof rafters so you'd probably need to lay some 3/4" OSB for the floor. I like to lay a layer of
Panda Film, white side up, on top of OSB floors as I find this makes it easy to keep the floors clean without any funky wet wood spots if I spill something. You could also lay some peel and stick tiles if you wanted to get fancy, also to make floor cleaning easy.
Not sure how high your gables are but I would imagine not more than 5 ft at center beam. You're probably going to want to use SCROG or SOG method because of the low roof issues you'll probably have.
Before you close up your walls I would suggest you run all electrical and water lines you're going to need up there. I'd run PEX for water as, IMO, that would be the easiest way to do that without having to cut up walls. I'd tap two 20 amp dedicated circuits off of your electrical box. One to run your A/C unit and the other for lights and whatever else. Obviously this only an example since you could be hanging more lights or whatever other variable that I'm not aware of.
