a: many, many people have built clones of the Dallas Rangemaster. Some have even gone into business making and selling them.
b: they sound like a treble boost, and have enough boost to push the input tube of a tube amp into overdrive; Tommi Iommi and Robben Ford to name two have used them.
c: The original Rangemaster was build out of common-as-dirt parts. There's no reason a clone can't be as good if you do a good job building.
Go re-ask the question over at the forum at diystompboxes.com. You'll get a lot of help and testimonials.
And go read the Guitar Effects FAQ at GEO ( http://www.geofex.com ) for more info on making your own effects.