Part of the voluntary difficulty of this project is the game of scavenging. Buying stuff is a kind of defeat. Of course you can buy a whatever for whatever. But justification for liberty in buying parts very quickly converts to warrant for simply buying a CNC mill that will be better for less than the ultimate cost of completing an inferior project paid for in small increments of direct cost + time not actually using the better machine.
On the other hand, working through building a whatever from temptingly cheap/found parts is a great way to learn what is the value of a more costly well-finished product.
Making stuff on the cheap is a hobby, not a profitable enterprise. If not fun, then likely not actually economically justifiable. Assuming first world circumstances where everyone specializes in some way to generate income more efficiently than than they can avoid expense by not paying others to do what they do more efficiently.