NCstar makes really cheap red dots that are great for nerf. Some of the "rails" on the nerf guns are almost to weaver specs, which is annoying because you have to modify it so it doesn't fall off when they could have used like 1mm more plastic and it would be a snug fit.
Typically I take the whole thing apart and paint it black, sometimes add a stock. If you paint it, taking it all the way apart first makes it look nicer, just tape over the screw holes first so they dont get filled with paint.