Welcome to the forums, Sam!
Just a thought re your K100D - have you tried cleaning the camera's battery contacts? Some recommendations say to use a pencil or pen eraser, but a pad with a tiny dab of metal polish seems like it would also be a good option. If the battery contacts are dirty, the resulting resistance might be causing your problem. Of course, it may just be that the camera is at the end of its life, but worth a try, perhaps. Another option is to try Eneloop batteries... Several older Pentax models have problems with regular dry cells as they age, but using Eneloop rechargeables brings them back to life.
Otherwise, for your next camera, I would recommend the K-70 over the K-S2. Both are fine cameras, but we've seen a small number of reports of the K-S2 developing a problem with the lens aperture control. This was more prevalent on earlier models such as the K-50 and K-30, but we've defintiely heard of a small number of K-S2's exhibiting the same behaviour. Not many, though. Still, I'd recommend the K-70 as we've yet to hear of any such problems with that model, and in any case it's a real jump up from the K-S2 in features and image quality. If your budget allows, that's the better choice of the two, IMHO.
As already mentioned, a used K-5, K-5II, K-5IIs, K-3 would also be an excellent choice, with "professional" build quality and very long shutter life (although they lack the tilting screen of the K-70). The K-5xx models are very affordable in the secondary market, and if you find one with a relatively low shutter count in nice condition, it should last you an awfully long time. The image quality is outstanding, too. The same applies for the K-3, though you'd have to pay quite a bit more (still less than a brand new K-70, however!).