Craig,
My opinion is that an airbrush is an investment. If you're planning on staying in the hobby for a while, then it's worthwhile getting one and start using it. It takes a while to get the hang of it, so the more practice the better. If you're just planning on building one or two kits, spray cans can do a pretty good job.
My opinion is to paint it how you want it. The color you pick will likely be 'close'. If you want a 'contest-winner', then you might want to research the colors a bit more and get something more exact.
Your thoughts on spray vs. brush are pretty accurate. I generally spray as much as I can, and do various touch-ups and details with brush. You can spray 'everything', but that generally requires lots of complex masks and other things that can really be time consuming. Use your judgement and build it the way you want.
Sounds like the project is going to be a 'BIG' one, so spend some time looking at the instructions and figuring out how everything is generally going to go together. Sometimes it's better to assemble things 'different' than what the instructions say. Focus on building several 'assemblies' rather than one big model. At the end, you just put all of the 'assemblies' together to complete the kit.
Do some research on the net to see if there are 'build-up' articles about your kit. They can be handy and give you a head's up on future problems you might have.
M.