You've gotten some excellent and extended answers to your questions already. I'll explain why UBI and I don't allow students to use stainless when building their 1st frame in class. The simple answer is that it is too difficult for beginners to do! It has a narrow brazing temperature range, it requires way more scrupulous cleaning and once things goes south you can't recover. Its like learning to swim by being thrown into the deep end of the pool. The sensible approach is to start simple and as one gains ability and confidence, increase the difficulty. A first frame should be successful and not one that has a high chance it needs to be cut up because brazing skills aren't up to par. Regular steel has a higher tolerance than ss of absorbing rookie mistakes.

Another thing I'll mention is that all the money you spend on your education and equipment isn't going to be totally wasted if building frames isn't for you. You come out of class with at least a custom frame fit just to you and the equipment you bought can be resold. A nearly new Anvil fixture doesn't loose a lot of value.

The question of equipment costs has for already been well answered. I'll just put it into different words. Tig welding requires accurately mitered and precisely placed tubes. This means expensive tooling and fixturing. With lug brazing hand mitered tubes can be spotted together and moved into alignment. For example to insure that a rear wheel centers exactly in the plane of the frame, hand mitered chain stays can be moved in and out of a socketed bottom bracket shell until a wheel centers. With tig welding, the miters have to exactly match at precisely the right length. Because brazing requires less expensive equipment, it is the logical place to start.

I'd also mention that the more education/training you've received, the fewer frames you need to make before they are acceptable to the public. That is one of the reasons my most common class is 3 weeks long. Those that are self taught with manuals and online help have a much longer learning curve. I'm forever thankful for having had the opportunity to learn from a master in Europe and am proud to show off all my frames from the beginning. PM or email me and I'll send you a long list of reasons why to take a class. Just like every framebuilder isn't equally skilled, neither is every teacher and class equal. After class you will have a much better understanding of what it takes to build frames and whether you have the interest and/or ability to continue. It will answer so many of your questions.