Originally posted by Rondec FA 77 or one of the 70-200 f2.8 lenses out there would fit the bill. You could probably swing a used Sigma or Tamron for the amount you are looking at. To me, the Pentax DFA *70-200 is worth it, but it is a very large lens and the price tag is high as well.
As far as the 50 recommendations, to me they are a bit wide for portraiture on a full frame camera. Not that you couldn't shoot full body shots or waist up shots and crop a bit, but classic portrait length is usually 85-ish mm. The 50s work better on APS-C for that purpose.
I think you right 85ish looks much better on full frame ,I saw legend old fa 85mm at b&h it s around 800$ I just worry about it s old lens but I heard many good things about this lens and I think it s the best 85mm for pentax so far.