The thing is, if you are building for other people, you have to do a good job in a reasonable amount of time. This really means you need a certain amount of equipment. As a minimum, you need the ability to check alignment. Pretty soon, the investment starts to add up. Your processes have to have an element of certainty that good tooling gives you. I really don't feel like making a list for what I think going into business would require, but it adds up quickly.
I think when I started we were a little luckier. Some of the shops I see from lone framebuilders are better equipped than Trek was in the '70s. It wasn't all that easy to go out and buy bike specific tools, importing was a drawn out process. So we improvised and built our own. You can do a lot with angle iron and allthread. I think one of my best investments back in the '70s was a box of 2" long 3/8" bolts and matching nuts.