What materials and spacing should I use for ceiling joists/strapping in this conversion?
I'm assuming since this question is over a year old you've probably found the help you need, but i'm here so here's my answer.

The framing should be a consistent 16 inches from center to center of each board, if your rafters don't allow for this sort of layout the MAX i'd suggest would be about 18-20 inches, any more than that and you'll run the risk of the drywall sagging. Make sure to run a perimeter strap all the way around to secure the outside edge of the drywall as well as give a place for any trim or molding to attach to. Another tip I'd suggest is to use a chalk box to line your rafter framing and drive a longer screw through all layers for added security. As for the insulation avenue, spray foaming will do a nice job however there's a lot of risks if done improperly. Inside and outside temps MUST BE over 65 degrees, any colder and the foam WILL NOT CURE properly and off gas indefinitely causing  very substantial health risks. If you spray it yourself make sure to rent an outside air respirator and full faced mask, fumes are no joke, they'll beat you up in a hurry, and unfortunately most manufacturers imply on the product that a dust mask and eye wear are sufficient, but its no where close. I'd suggest rock wool, slightly cheaper per square foot, a comparable r value, plus the added bonus of being able to dry out if ever dampened (foam traps moisture)

Save yourself the back ache and rent the drywall lift, most home improvement centers and building supply company have them available you need only ask, the prices are usually very reasonable however if you've ever hung 5/8 rock over head before, even one piece, you'll know the price is very worth it!! 

hope this helps and happy building!!