“What is the minimum purity required to make weapons grade plutonium?”
Actually, There is no weapons grade plutonium as it is all plutonium. Uranium has two isotopes and enrichment seperates the the two. U235 is the good stuff but makes up about .07% of naturally found uranium. Thus enrichment is required to get the good stuff.
The bulk of natural Uranium is U-234, U-235 or U-238 but there are a couple more trace isotopes, making five in total that are naturally. Plus another couple of dozen or so that occur as fission byproducts.
Lecture on nuclear weapons shows a slide listing weapons grade plutonium:
https://youtu.be/zVhQOhxb1Mc?t=2465