It's certainly possible to do, but it's rarely worth the cost. Brazing on new cantilever brake mounts will be a fairly expensive process, and will also require a new paint job (the torch does wonders to paint). Same goes with the fork, although it will be even harder to find someone willing to mess with a fork (as a catastrophic failure is very dangerous).
Given this, I don't really think it's worth the effort you would have to expend. If you're just starting out, buy a nice old road bike, they usually came with relatively long chain stays and clearance for moderately wide tires. It won't be perfect, but it will be within your budget, and you can tour the hell out of it before spending more money.