These new metrics have nothing to do with Google and if you want to know about the QUALITY of a site, they are worthless. Their metrics are based on backlinks...nothing else. This is why 99% of sites with thousands spam links have insanely high metrics for a sites that are pure garbage.
It's actually impossible to get a PR of 3 using spam links alone because Google's PR algorithm is pretty solid. In order to get PR, the inbound links must have PR in the first place. Spam links don't have PR, simply because they cannot attract links.
If an expired domain has a PR of 3 and Wayback shows it was a real site, with real content and the links that gave it a PR of 3 are in place, there is no reason to assume it's PR will have dropped.
Actually, it's PR will probably have increased because the legitimate sites that are still alive will probably have attracted more links over time and in turn that PR will be passed onto your domain.
If you are buying old domains based on those 'new' metrics and expect them to be an accurate reflection of how Google rates them, then you might as well throw darts at a list of sites.
I've said it before and I'll say it again. Google does not use third party algorithms, it never has and it never will. So if you expect Google to look at a domain and say, "hey, Ahrefs and majestic rate this site with their clunky metrics higher then we do, we better move it up." your gonna be waiting a long time.