I would find lights that don't required to be hidden in a ceiling cavity, pendant mounted cans or something pretty.
Rip the old roofing off and install some roof SIP's (Structurally Insulated Panels), cover with a white rubber membrane roofing, or metal roofing, or light colored shingles if the pitch is warranted by the manufacturer.
LED light fixtures, smart light switches which know when you are in the room will save you a lot of money in the long run.
ALTERNATE: If you really wanted to seal the roof up tight, consider spray on foam insulation and finishing as discussed. There is some soy based stuff out there which you should research before deciding.
I went to a lecture recently which provided lots of information. This is the main speaker's website.
http://web.me.com/kellygrocoff/missionzerohouse/Home.html
Here is a local (to me) supplier of the smart switches.
http://www.ferguson.com/