I spent almost a year looking for a used HST Deere and finally just bought new one. I went to more auctions and reponded to more ads then I would like to admit to. The time I spent on the road and looking was all in vain. What I found in the area around Rochester, New York is that the HST 4000 series in good condition sells for about 80% of new. The 855 and 955 sell for maybe as much as 70% of the 4000 series with the same equipment.
If the tractor looks like it has been really used then I believe you would be better off buying a new one. The price is tuff but you have a new machine and a dealer to stand behind it.
I paid 19,100 for a 4300, HST, 430 fel with heavy duty bucket, 3rd hydro valve, mid-pto, the better bush hog rear cutter, heavy duty rear blade with skid shoes and a handfull of other add ons.
My guess is that I have almost $4000 of equipment not listed on the 855. That is exactly what put me on my new 4300. Buy a 4300 and you will not regrete it.