Note: the knee wall would be the ideal place to stick your outlets, rather than on the sloping ceiling (if you let that sloping ceiling go all the way to the floor & left those vent pipes visible). Tip: when you're building that knee wall, after you cut all the vertical members, but before you do any nailing things together, just stack them on top of each other & drill down through all of them at once for running your electrical wiring. Makes life a bit easier later on, rather than crouching down & squeezing below the roof to drill those holes one at a time.
Maybe not for the pros, but I found that angle between the kneewall & the ceiling to be a bitch to get a perfect looking seam when I finished the drywall. Though, there's now a flexible corner thing (some sort of plastic) made for just such a place. Makes it a hell of a lot easier - I wish I had known about that product when I refinished my attic - took me 5 or 6 coats of mud plus a shitload of sanding before I was completely satisfied; I can do a normal wall in 3 coats very easily.
If I had to do it, I think I'd insulate all the way down the roof (with the rafter vents in place down to the soffit vents), starting from the level part of the ceiling. Then, prebuild the knee wall first, planning for it to go up against the ceiling drywall. As I built it, I'd make sure it was ready to run the wires through. But, I wouldn't fasten it to the floor immediately - I'd slide the ceiling drywall between that and the roof rafters. That way, the knee wall holds up one end of those awkward to handle pieces of drywall for the ceiling. Plus, you can just cut them 2 or 3 inches long & not worry about it. Stick in the boxes and wiring for the outlets. Then, slap your drywall on the knee walls & other vertical walls (some would need insulation.) It's a project that if you get that drywall to look good, is incredibly satisfying to see the results of and know you did it yourself.
edit: with the insulation on the roof & not on the knee walls, you could run the ceiling drywall as close to the floor as you can get it, then with a couple of removable panels on the kneewall, can turn the space behind the kneewall into "secret" storage.