Walther Dominator 1250 is already set up as pcp a from the factory. Plus it has a regulator which is working pretty good.
hammerli Pneuma - that is a Hatsan pcp from Turkey.. Stay away from those.
850/1250 guns are a bit better, it has some Taiwan made parts and Walther barrel. That is why they are accurate. But the receiver is made from two pieces from pot metal that is of not very good quality. Might as well you step up to an FX Whisper for a few hundred $'s more.
Also look at the Korean made pcp's from Evanix, like the Blizzard models, or the Sumatra guns if you want more power 60-70fpe., they are are great value, better then Hammerli Pneuma for sure.
In 500-600$ the Condor or Talon SS from Airforce are the best deals, in term of what you have, and what you can do with them. You get good power and they give you a good barrel.
Then I would look at the Korean's at this price range, Evanix Rainstorm/Blizzard
for $700-800 FX Whisper is the best deal going... and at $1K Air Arms S510 and Daystate Huntsman are the two best value rifles..
As far as the 850 store goes, Roald the guy that runs it knows these 850 guns inside out, and if that is what you like sure get him build you a nice 850 PCP conversion.
There is also Crosman Marauder @ around $400-450 it is also not a bad deal. But Airforce and Korean products would be better then it.
Also check this one ouhttp://wop.pyramydair.com/s/m/Gamo_DynaMax_3-9x50_RGB_Dot_Scope_Combo/2029
a very good buy as a first pcp..