The real question is, if I buy 80% of a 3d printer to be finished on my own, does it need a Prop 65 sticker?

(The answer is actually "yes, several".)

i think it can go further than that, such as circular scenarios, what portion of the item, is the part to worry about.

if a printing or milling job, or some combination of both, is split into many portions, until each portion is such a jigsaw puzzle, [perhaps literally] that it cant be filtered as its so non specific in form, that it could be anything.