it's not the metrics that matter the most with expired domains, but their content history and backlinking profile. I mean, if you find an expired domain with DR 80+ chances are that you've struck gold anyway because high DR usually implies very good (strong) domain. But you don't let such domains expire, so I assume you're alluding to more approachable domains.
If that's the case, the 2 most important things to look out for are - like I said - content history and backlinking profile. Oh, and redirects, too. You want your domain to have as few redirects as possible (preferably 1 or 2 at most).
Content history... you need to check what type of content (or rather, what niche) the domain had when it's been first registered, and then check the content throughout the years until its expiration to be consistent with / closely related to the first content. To check this you would use the WBM at this URL...
http://web.archive.org/
Just pop the domain you're interested in, into that field and right click on each one of the dots that WBM has available for that domain and check the content to be consistent / relevant throughout the entire history of the domain. You also want NOT to see content in any Asian language. Why? Because google doesn't rank Asian content in non-Asian markets. Besides, many of the expired domains have been purchased by Asians (Chinese in special) and redirected to all sorts of shady offers / sites, and google have marked those domains as being "bad"... for non-Asian markets anyway. Additionally, you might probably want NOT to see any warez / piracy / adult / gambling / pharma type of content in WBM either. I mean, if the domain's name and initial niche is in one of these shady niches then you probably do want to see that type of content throughout the entire history of the domain. But otherwise, not
As for the profile of the backlinks, the most important things to look out for are the backlinks' number and quality.
EDIT: I forgot to answer to your other question, the one about platforms to find good expired domains.
Well, I don't know which one is the best (probably godaddy auctions despite of me hating goaddy with passion), but other people recommend Spamzilla, DomainHunterGatherer, expireddomains.net... and I also used Scrapebox in the past and it's been pretty good at finding quality domains.... apart from these there are also dropcatch, justdropped, domcop, and a few others that I don't remember.
