You get what you pay for when it comes to woodworking tools. If you go the hobby route, woodworking can get very expensive. Good, new hand tools (planes, saws, chisels) are pricey but worth spending the extra money. Here's a link to a miter box that is probably decent. This company (Lee Valley) usually carries good stuff.
Nobex® Professional Miter Box - Lee Valley Tools
As far as the Dewalt, should be good to go. No reason for a special blade for PT wood in my experience. The stuff you get at the box box stores is usually pretty "wet" and prone to warping and shrinking.