If expiring domain are listed in major directory, then easy have traffic from them, also still received a bit page rank, especialy from DMOZ, where in some categorys high pr.
If you will buy with namejet then you can have registered with several registars.
Transfer of new renewal expiring name is same easy like each other domain.
moz Rank its another factor after page rank, which talking how trusted is domain, how trusted have backlinks.
Orginally moz Rank been created by and search engines not use it straight, but moz Rank can tell you about domain trust in eyes of search engines.
Old expiring domains are much better in compare to new registered.