Many frames have been built with very simple tools. No real need for powered mitering unless you're going SS or Ti. Besides the usual fabrication shop stuff tubing holding blocks/clamps are a nice thing that will be used a lot. If you're building infrequently and have a flat surface that you're good trusting I suggest a tube holding set up based on that surface. Alex Meade makes some. The only real reason to have a formal jig is to quicken the build time. Many people will wait till after a few frames before shelling out the $ for a quality and useable jig.

My I ask where you are located? Some aspiring builders will take a building class of find a mentor to help them. Andy