This is an example of what I'm seeing on Gumtree in Oz. This is still over budget and it is manual focus, but it gives you some idea of what things are going for...
Samyang 85MM F1.4 Lens Pentax/Sony E Mount | Lenses | Gumtree Australia Melbourne City - Melbourne CBD | 1163328834
FA 28-105 f/4-5.6
Not the best bokeh and not very fast but AF and covering a good range for group shots and portraits and well within budget...
https://www.gumtree.com.au/s-ad/manning/lenses/pentax-fa-28-105/1168934430
DA 17-70 - SDM scares some people (Crop Only)
This lens is very nice at both ends of the zoom range. A touch slow and a touch less long than ideal but at 70mm it can make great portraits and it opens wider for group shots. constant f/4
https://www.gumtree.com.au/s-ad/homebush-west/lenses/brandnew-pentax-smcp-da...ens/1168281976
Sigma 70-300 f/4-5.6 (Might be Crop Only)
Slow, long and longer, Bokeh could be good with higher working distance if you can work that way effectively.
https://www.gumtree.com.au/s-ad/clarinda/lenses/sigma-apo-70-300mm-f-4-5-6-d...nt-/1155643609
Super Takumar 135 f/2.5 (m42)
You would need an m42 adapter, and it is manual focus. But good bokeh and great isolation would be yours for peanuts...
https://www.gumtree.com.au/s-ad/point-cook/lenses/pentax-super-takumar-135mm...ase/1164968082
F 80-200 f/4.7-5.6
Long, slow, bokeh only if you really push the distance to subject and push back the background - but light and useful and cheap.
https://www.gumtree.com.au/s-ad/sydney-city/lenses/pentax-mount-k-lenses-for...st-/1167346248
If it were me - I'd look long and hard at the manual focus options like the 135 f/2.5 - but if AF is a must the 28-105 and 80-200 options are good possibilities unless crop only lenses are OK in which case the 17-70 seems like a good bet.
With your budget it will be tough to find an Autofocus portrait lens without compromises.