Will sealing off rafter spaces and unusable spaces help my attic insulation plan?
First of all, foam and plastic are fire hazards. Don't turn your attic into a giant match head for your house. 

It sounds like your attic ceiling is not yet insulated, so that is where you should start.  The fiberglass stuff goes on sale pretty regularly at the big box homecenters. You want the paper backed stuff. The paper makes it easy to staple to rafters and it will stay put once it's up. Your roof requires some air circulation, so before putting up insulation between the rafters you will need to put up ventilation channels. The big box stores sell them, though I can't recall the brand. If you can't buy it all at once, do it a roll at a time until the whole ceiling is insulated. 

When you have the entire ceiling insulated you will probably be surprised at how much warmer it is.

But your job isn't really done at this point. The craft paper backed insulation must be covered with drywall in order to pass any type of fire code (I'm not a fire code expert, but the insulation says so right on it). So once it's insulated, put up drywall. No need to plaster or paint yet, jut get the insulation covered. Start by making a peak ceiling and then fill that whole space with fluffy insulation. Then would you way down until you're ready to do the kneewall. Drywall is also fairly cheap. Just requires time and probably a friend to install.

If you close off the space first and it still is not warm it is VERY difficult to insulate after the fact.

Good luck!