Climate control for garage?

[Izlude]

Hey again.  My garage gets pretty cold/hot during seasonal swings, mostly due to the garage door not being insulated.  Another issue is the particle board back door. It's not sealed at all (huge space on the under side from no threshold and the sides have gaps). The floor is bare concrete. I've got some r-13 in the walls at the moment. If I were to insulate the metal garage door with r-8 boards (then put heavy curtains since that door will NOT be opening), get a new backdoor (solid wood with threshold and sealed edges)  what else would I need in order to maintain a constant 70 degrees F and 40% humidity? Floor type? Hook into the home's air system?  Or would the air system not be an issue, and just use something portable?  Size wise, I think it's a lil bigger than 20x20.

[metallithrax]

Serious question, but why do you want to keep it at 70?  Do you spend a lot of time in there or is there something you need to keep at that temp?

[Izlude]

hiya,  yes it's a living quarter so i'll be in there most of the time.  i've also got some temperature sensitive equipment (musical and electronics) my pal's place has a converted garage, but it was like that when they moved in so they couldn't give advice or know how.  it feels like the rest of their house (never has extreme temps) they're able to keep a baby grand in there without issue.  not sure how this kind of conversion is achieved.

[xendrome]

Is the airhandler for the AC located in the garage? if so have an HVAC guy cut a vent into the outgoing airflow for the garage. Then as you said, insulate the garage door with fiberglass panels.