If you go with a MTB frame and successfully overcome all of the mechanical hurdles mentioned by Ronsonic and probably plenty more (700cc wheels for a 26" MTB frame?), in the end, you will look at the thing and still want a "cyclocross" bike.
Why not save yourself the hassle of doing it twice and just buy a cyclocross specific bike to begin.
Ebay... another person's mistake can be your bargain.