This can be tricky as the compressors changed in 86.5.  The key is to look at
the rear of the compressor where the lines attach.  The earlier style SD-508
had screw on type connections while a SD-508 in 86.5 had the non-screw on
type (do you have a better word for them?) that is basically inserted and
then held on with a bolt and a bracket pressing the fittings into the pump.
If you have this later style I am pretty sure that you can upgrade to the
SD-709 with no pain.  So check your connections and this will help you
determine if the used pump will fit.
Also drain a little oil out of the used pump and inspect it.  If it is very
nasty then, it might be a good idea to walk away from this compressor.

Hope this helps,
Ken Wilford (Certified Vanagon A/C Tech!  I am crazy and have the papers to
prove it!)
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242