What quantitive metrics would reviewers in order to be presented as candidates for peer review? I am looking for ideas and suggestions on when someone would have enough experience and expertise to be presented as a potential peer review candidate. 

I am specifically looking at using review history data as a source and keywords search to search that source. So a solution could be related to attributing keywords. But don't let that lead you too much, this is just my current thinking.

The configuration of such a process of when does someone becomes worth showing as an expert of candidate is main area I would like suggestions and idea on.
If I had to find peer reviewer for a field I have no knowledge of, but a lot of data, I would first come up with a citation graph and look at all papers near to the paper that should be refereed. 

If I had a list of reputable journals, I would filter that paper list by this list. The resulting paper list can be scanned for authors. Now you can compare key words of the important papers of the authors to the key words of the paper. This allows you to come up with a list of persons that may be "experts".