2nd hand Mac Pro: what to look for? Want later models

http://store.apple.com/us/browse/home/specialdeals/mac/mac_pro

Stick with the 2010 model. "2012" just was a slight change in default processor offered.

Nothing older than a 2009 though. You save on used 2009s if you find one.

Apple-refurbished are in pristine condition They are indistinguishable from new. They are eligible for AppleCare extension under the same terms as new.

Additional information for completeness:

The 2008 model and previous are Very Different from the 2009 and later. The 2009 and later can accept much more memory and that memory is substantially cheaper. The 2009 and later with the same number and nominal speed of processors approaches twice as fast in benchmarks due to architectural differences

So if you expect to do compute and memory intensive modeling, Photo, Video, or Audio editing, limit your hunt to the 2009 model and later.

If you are doing general work that does not involve large memory sizes, any model Mac Pro will work for you.