The Pentax KP is currently 746.95 dollars at BHPhoto and Adorama. The K-5IIs can be had used for about $250 as luftluss said.
The differences:
K-50 to K-5IIs - in terms of inage quality, the difference is that the K-5IIs does not have an AA filter so the images will be a bit sharper. It's also 14-bit RAW so the images will have a bit more "room" for post processing. That is it. The other differences are mainly a more pro body and some extra features that the K-50 does not have.
K-50 to KP - 24MP will give you significantly more room for cropping. It doesn't sound like much but it is. Of course your lenses need to keep up - you can't just use a low quality lens like a DA 18-55 on it, and think you can crop to your heart's desire. Newer lenses are recommended. What do you currently have?
The K3 is another option. It also has 24MP, no AA filter and 14-bit RAW so your images would improve tremendously. A higher burst rate than the KP as well. Used prices go for about 350-450 depending on the condition. That would leave room for a DA*300mm f4 if you don't have one yet, in your budget - they can be found easily right now for 650-700 dollars in the used market. You can go whichever way you'd like but that is what I would do if I were in your shoes with your budget and your stated photographic objectives