I always thought it was cheaper to start with what you wanted. Been too long since I read Sheldon Brown's site; it goes over all the various bits. Stem diam, bar diam, etc. I'd be sorely tempted to shop CL and maybe used bike places and see if what you want is already out there. Might not be the cheapest though. But you may have fun finding the right exact parts to pull off what you want.

As for tires: you could mark the forks and chainstays where the tire is, remove wheels and measure the width. Or carefully measure as-is how much clearance is on each side of the tire, and add that to the current tire width. If you're not hitting mud then you should be ok to get just about that wide.