Attic subpanel

I am finishing out about 900 sq ft of my second floor that is currently being used as attic. The framing is complete. I will have a 18.5'x30' theater with a wet bar and a bedroom in this space. I currently only have a 20 amp GFCI outlet and some keyless lights that are on a shared circuit with my second floor lights. I would like to run a 90 amp sub panel into an unfinished storage space adjacent to the theater. My plan is to run the wire up from the subpanel to the ceiling joist, down that to the edge of the house, out of the soffit and straight down the side of the house, then punch into the crawl space and come out right under the existing 200 amp service panel. The section from the soffit to the crawl space would be run inside of 1 1/2 PVC conduit. Do I need a J box at the soffit and at the bottom when it goes into the crawl? My big question is what size and type of wire do I need for this? I might only wire up 80 amps but I would like to leave myself some headroom so just pretend it'll be 90 amps now.