I have the K-5 and I love it. There is something (which I'm sure better photographers than I can explain) peculiarly excellent about the sensor that was chosen for it and the image processing behind it, that keeps the entire K-5 series being some people's favourites even in light of every other APS-C Pentax body that came after it, with nominally better AF and nominally improved (higher MP) sensors.
That being said, the later iterations (the -II and -IIs) had significant improvements to their AF and a few other things (e.g. removal of the anti-aliasing filter in the -IIs) without losing that special sensor something, but they came out just too late for me to buy them instead of the base model (I was replacing an earlier Pentax model that had died), and when they did come out, I couldn't justify the upgrade cost in the context of the limited amount of shooting I do within my profession (as opposed to personal fun stuff). You are almost certainly better off with a K5-IIs than with a base model K-5.
I have heard it said that the glass matters more than the body; even a 6MP sensor can deliver fantastic images, because it's all they had in the dawn of digital and people did fine work with it. So on that basis, a used K-5 plus lens(es) may well be the way to go. But
if the kit lens on that KS-2 was the DA 18-135... that is a whole different kettle of fish, because therein you would have a weather-resistant (but not waterproof and certainly not swimmable) package out of the box, which would be one-lens-does-all for 95% of your holiday needs, and many others as well until you could sort out where in the focal length range your shots were clustering and which other lenses you should start saving up for. And the KS-2 is, from everything I've heard, no slouch of a camera (even if it is not a flagship-level body).
It is possibly the case that you might be able to find (e.g. on the Marketplace here) the combination of K-5 series body and used DA18-135, and if you could then that would be ideal.
The next lens you should consider after that is a fast prime, e.g. 35mm or 50mm, for shallow-depth-of-field shots and flashless low-light work. How prepared are you to go manual focus for those "artsy" shots? Because then your options
broaden enormously and the price drops way, way down in many cases.
Do not forget in all this, however, how useful a small but strong tripod can be (when you are not "on the fly" and conditions permit it).