Growing temp- 65-68 night, 72-78 day
Cellar is the best palce to start because it is already insulated
Drywall is all well and good but have fun taping and mudding, priming, and painting. Or save yourself the hassle and use shower wall board that is alreadyu finished in gloss white on one side and hangs in minutes.
You need super vents. My attic has windows at the gable ends and thus space to run a/c. You WILL need ac regardless of whether it's only 80 degrees outside because yup the roof will get HOT and also heat rising from the rest of the house is a pain.
Attics are all good but enjoy your heat problems... I know I did! And my crops suffered badly as a result.