100-400 USD depending on condition, accessories, box, manual, etc..
250 USD
You will need to contact Ruger for a correct answer
Ruger has sn data on their web site.
100-300 USD or so
Go to Ruger's web site and request a manual.
The original P85 had a design flaw which had the potential to result in an accidental discharge. The Mk II corrected this design flaw.
25-350 USD or so
If you are referring to the 33 round double stack magazine - I know it will fit the Ruger P-89. If the P-89 mag fits the P-85 your answer is YES.
Ask you local police to run the serial number. It has been reported it will show up.
Of course it depends on condition, whether its a MK-I or Mk-II, etc. but the value shows around $300 average.
"Best" is subjective. A really good definitition of best is, the ammo that functions the most reliably in your particular gun.