At our local building materials dealer, you can set up an account, even as a private builder, and then purchase throughout the entire construction process. This also gives you a few percent discount and the invoices come monthly. Quite practical.
Do you have that much time to handle the shell construction on your own? My father did that once as well, but he was in the military; at that time, the entire squadron was relocated and many comrades were building at the same place at the same time, and you always helped each other. Besides, duty ended at 4 p.m. at the latest and the barracks were just about 2 km away from the construction site. And if there was nothing to do, the commander sometimes turned a blind eye and deaf ear when he knew that the family men who were building had already left by 3 p.m. But even then, it was an immense effort. And as I said: people helped each other. Back then, detached houses were built without a crane, but the roof tiles were brought up with a conveyor belt that a nice farmer lent (and which was otherwise used to transport straw bales into the barn). All the colleagues stood there and distributed them on the roof. For larger projects, everyone met at the construction site concerned and helped together. That was how it worked then.
What I want to say: don’t underestimate the time required and how many hands are needed in some situations. Doing it alone with just a bricklayer will really be a challenge. How fast should the whole thing be finished? If there’s no time pressure, of course it can be done more relaxed. But the requirements today are also a bit different than 50 years ago. Back then, no one knew of an energy saving ordinance or the blower door test.