By the time you replace the back wheel for a freehub style and do the front to get disc brakes if you add up the costs you'll find that you're better off to just sell the Boulder and buy a slightly upscale model that comes with discs.
I'm all for minor upgrades but when you start doing the big stuff like this it's best to just stop and admit that you've got the wrong bike and it's time for an overall upgrade. Otherwise you end up with an investment that would have gotten you a model or two upscale and it's still "just" a Boulder and you won't get your money back later. Bin 'dere, got the shirt...... It cost me mucho bucks.