if i buy dropped domain do i get the PR, alexa and other stuff related to domain or all thing removed as new owner buy the domain ?
From my limited experience, you don't get the PR, but you could keep backlinks, which might benefit traffic if users click on those backlinks. Problem is, however, a lot of dropped domains have faked PR and their backlinks are spam, so it doesn't benefit you in the end.
Use a backlink checker to see whether these links are spam or genuine links. In the situation that they're are genuine they you're onto a winner. Re PR it could really be anything, I've heard different things from different mates in the industry, some say yes, others strongly say no. Any advice here?
I wouldn't worry about the PR anyways, the more important metric you need to look out is the DA (domain authority) as it reflects the power of that domain to rank in search engines. Aim for a higher DA and your PR would not really matter (information has been going about PR back and forth from Google for a long time now, but they generally seem to be abandoning this metric....or so they claim)
What do you think about expired domain with PR4 PA28 DA17? all links looks natural. How much I will benefit If start building article links in this domain to my money site?
It looks quite good, but before snapping it take a deep insight at the whole domain history (by using Wayback Machine) because Google tends to consider the entire lifecycle of a domain. If it has been used for spam purposes, don't take it.