This is a fascinating discussion. One thing that is confusing the debate, IMHO, is casting it as a binary question - hand built, or not? It's a continuum.
A friend of mine and I have a slow, ongoing project of smelting iron from local ore in a so-called "Aristotle furnace." It's slow going, a fist sized lump at a time. My dream project is to make a knife for a friend of mine out of ore from his town. The number of labor hours will be immense, but I'm really doing it for the experience. Will my hand-smelted knife be functionally or aesthetically better than one made from commercial 1084 steel? Mmmmm, doubtful. But it will have a better story behind it. And I'll have to tell that story for anyone to notice.
So how many of you lightweights are smelting the ore for your rifle barrels, hm?
Ok, so not even the big name gunsmiths of the 18th and 19th centuries did that. They made barrels from bar iron if they had to and bought them if they could. They made locks if they had to and bought them if they could. Or they built the parts that "mattered" and bought the parts that didn't, whatever their standard for "mattered" was. Any individual gunsmith of the classic era probably slid back and forth along the scratch-built to kit-built continuum depending on economics and circumstance.
The value of totally scratch building things is in the mind of the builder and the customer. There is something wonderful about an artifact with a lot of genius and sweat in it. I love a thing with a good story behind it. There is also something wonderful about a rifle that balances well and shoots fast and straight. The two don't necessarily overlap. Be honest and let the customer decide how much money he wants to spend and how far along the scratch-built continuum he wants to go.
I should note that when I was a teenager I built a pistol from a kit with the benevolent help of an old gunsmith named Henry Palmer. It took all the patience I had and more. Since then I have been in awe of anybody with the patience, knowledge, and fine motor skills to build even the most complete kit.