High quality isnt really the expensive part, Its setting up to do it in a particular way that gets pricey. The base costs are some simple hand tools, and a workstation, and vise; a joinery method, which requires a torch or welder, or both; Frame tools to clean up threads and ream various parts. You are looking at a few hundred to a thousand for the first catagory, 500-1000 for all new in the other two catagories.

I think a mill and a lathe are worth having in a shop, though not necesarry to build a frame. However the combination of metal joining tech, and machining tech, means you can build just about anything else you need from fixtures, to tools, to unique parts for your frames. So it may be worth it to get those items early before you have spent all you money on the things they could build. On the other hand, you can waste a lot of time on those things, so it depends on how soon you need to get going.

I cant believe for a minute that bike builders insurance will cover you. For one thing you may build a few frames and then decide it isnt for you, do you renew the insurance for ever to keep your coverage alive. You need a good insurance, legal, and asset strategy (like being at the lifecycle point where you dont have any) to be safe.