Welding the brake mount onto the Paragon flat mount dropouts is not a difficult thing to do. I just finished building a frame with them and I fusion welded all around the beveled edge of the brake mount. I used filler to weld around the top of the mounts. The only difficult part was getting the torch into the back side of the rear mount. It is probably easier to weld the mount on before building the frame to have better access for welding and avoid any potential distortion that may cause misalignment.

If price is a concern and you prefer flanged dropouts, the Allotec flat mount dropouts from BikeFabSupply are probably the best option. I wouldn't worry too much about the derailleur hanger. It would take a lot to break that steel hanger.