Aside from the k-3 (with it's greater number of autofocus points), all pentaxes (aside from the k-500) should display more or less the same information through the viewfinder (since the focussing system is effectively the same, aside from speed improvements with the newer bodies). The k-r however uses a penta-mirror, rather than a penta-prism found in all newer bodies.
In simple terms, the viewfinders in the k-s1/k-s2/k-3 are the biggest and brightest, the k-5/k-30/k-50/k-500 viewfinders are slightly smaller (but not by a huge amount), and the k-r/k-x are noticeably smaller and a little bit dimmer.
Aside from ISO, shutter speed, aperture, and the focus point, there really isn't any other information you need (all other settings that can be fixed later with Lightroom and raw files).
The larger viewfinders of the k-s1/k-3 will help you to visualise the focus point, and to frame the image better though.
I can't help but feel that the k-s2 is probably the camera you want to go for (that or a k-3).
The difference in image quality between the k-r and k-s2 is huge (I found the k-5 to be only a small upgrade over the k-x). The k-s2 also has numerous dedicated controls that should give you much more control over the camera than you have with the k-r.
So I'd recommend narrowing the choices down to the k-3, k-s2, or the k-5iis.
Either of those bodies (new or second hand), will put a big smile on your face