by caulk, i assume you mean silicone, which is what you should use if you build it out of glass. any silicone is fine, just make sure its pure silicone, and doesnt have any mold preventors or anything in it.
i would suggest you building the sump out of acrylic, since i would assume it would be easier. if you do choose acrylic, then you should use some sort of acrylic cement or bond.
im not quite sure, but hopefully someone with more experience with building tanks will chime in for ya.