Hi,
I think that the approach of spraying foam under the roof rafters, and making the attic space part of the conditioned space of the house is a good one if done carefully.
Seems like it would be a time consuming job to block all the current attic ventilation carefully.

If you use closed cell foam, I don't see why there would be any moisture or condensation problem with the roof sheathing.
Some of the people who make roof shingles don't like insulation right under the sheathing because it makes the sheathing and shingles run hotter and may reduce their life -- some people think this is bunk.

If you have ducts or AC equipment in the attic, moving the insulation line up to the roof rafters will make your AC and duct system more efficient. The FSEC has some papers on what kind of saving results from this.

Have you thought about just blowing additional cellulose insulation on the attic floor? Seems like this would be a lot cheaper, and may accomplish about the same result?

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/97/970504.html

Gary