When I turned my hobby into "for pay" with an 07, I mentored briefly with a lifelong smith. He had a tiny garage shop, packed with two big CNC BP's and a Hardinge. I "subbed" some work that demanded precision that I wasn't comfortable with in the beginning to him....
Guy's a real machinist- or so I thought. Decades doing work for Troy, Police dept.'s, you name it. Custom 1911's- absolute wizard.
Gave him a rifle that needed the muzzle threaded for a brake. Picked it up, and as I'm packing it up to send to the customer I can see the brake VISIBLY crooked on the muzzle. Scary. Took it back and showed him- all I got was an "oops". One of my first customers, and would've been a disaster- or worse- had I not caught it.
I learned then, that despite all that you hear about years of "experience"- sure as hell doesn't mean someone is competent. Could he do the work? I'm sure he could- but the fact that he was sloppy, and could've injured someone because of it, was an instant "never again".
Was it a fluke? Dunno. All I knew, is I decided that day that smithing isn't rocket science. It's about precision machining, having a sound understanding of the tasks needed and knowing how to execute them flawlessly. After that, I got over my "I don't want to risk effing up the customer's rifle" fear and dove in- slowly. I swear I sweated bullets worrying about everything I did for the first two years. Slowly, carefully, deliberately.
I made the decision to specialize in long guns. Even that- is a big, big arena. Bolt guns, semi's, levers, shotguns...
Vast majority of what I do is bolt guns- because I love 'em. Something about turning and chambering barrels to high precision brings a lot of satisfaction to me. I don't work on pistols or revolvers at all.
Being an "old school" smith is an enviable goal.
I think it ends up deciding whether you want to be a "specialist", or "jack of all trades".
In this age, I believe a "specialist" has a better chance of succeeding. In any case- buy a lathe and milling machine, plenty of "how to" videos online to get started. See if you have the aptitude for it, and more importantly, whether you really enjoy it. Standing in front of a lathe for hours on end can be tedious, which is what you do when you build custom rifles.
I work out of a converted 600 sf garage at my residence. If that's your plan you need to check your zoning- because BATFE will when they come out for inspection. Zoning restricts me from having walk-in customers, but I really don't care because all my work comes in via USPS or common carrier. I don't subscribe to the "tin hats" about BATFE storming and searching my home. I get calls from the local field office occasionally, about seminars or alerts (hurricanes, civil unrest)- the agents I've dealt with have all been professional and supportive.
I suspect they're more interested in compliance with retail gun stores than gunsmiths not involved in sales/transfers on a regular basis.