I'd agree that the best course is to upgrade the body.
One of the big changes in each generation of Pentax DSLRs has been better low light performance. When I went from the 6mp K100D Super to the 16mp K-30 it was a revelation. The 24mp sensor in the K-3 wasn't quite such a jump, but the KP was another big jump. A KP or K-70 would give you two or three stops better performance - 3200 ISO is really useable, and you can go higher on occasions.
@Normhead makes a good point about noise reduction software. There is plenty of high quality software out there now. Topaz is a popular option. If you rely on faster (ie wider aperture) lenses to get more light, the lenses will generally be larger, heavier and more expensive. These days slower, variable aperture lenses (e.g. DA 16-85, DA 55-300 PLM) are high quality and comparatively light weight. These lenses can be used effectively with modern bodies that allow for shooting at higher ISOs.
For 500 pounds, you should be able to get a K-70. I'd suggest looking for a bundle with a DA 18-135. It would be worth stretching your budget for this if possible. If you can live with a smaller focal range, there are plenty of other options of course. If you want a wide-aperture lens to pair with it (e.g. for portraits with short depth of field), look for an older prime at f2.8 or faster. For example there are plenty of good affordable 28mm, 35mm and 50mm primes that would fit the bill.
You haven't said what sort of shooting you do in low light. If it is indoors, I'd suggest learning more about flash photography - there are easy, cheap and effective ways to get good images using flash, either by bouncing it off the ceiling or walls or by using a "flag" to direct the flash beam so that it reflects onto the subject in a diffuse and flattering way. Have a look at some of Neil van Niekerk's video for ideas about this: ? Flash Photography Techniques - Tangents There are several good cheap flash units that could meet your needs. (Post a separate question in the flash section if you want more advice about this.) If you are shooting landscapes/architecture in low light, the first answer is to use a tripod and slow shutter speeds.
I see from your previous posts that you have an interest in wildlife photography. If that is the situation you have in mind, the answer is a little more complex. Definitely go for the body upgrade (modern bodies are much much better for wildlife), but the lens issue is more difficult.For wildlife you need longer lenses - 300mm is a starting point. Here you have to choose. The entry point for an affordable long lens with autofocus is one of the xx-300mm zooms, the best of which is the DA 55-300 f4.5-6.3 PLM (for which you need a more recent camera than the K-m). It's a good lens, but only f6.3 at 300mm. In low light, that means high ISO and/or slow shutter speeds, or adding flash (yes, this can work for wildlife too, but if you use too much you will get poor results). A longer lens option like the Sigma 50-500 or 150-500 is big and heavy; it has more reach but still slow aperture. Beyond that, forget about the 500 pound budget. If you want a wider aperture, you will need something like a DA*300mm f4, which is more expensive and heavier than the xx-300 zooms, but not a bad compromise. Or the DFA 150-450 which is bigger and more expensive again. There are long f2.8 lenses, but they are very heavy (and sometimes very expensive). There is no free lunch here. Have a look at the long lens thread (
https://www.pentaxforums.com/forums/122-lens-clubs/55946-300mm-plus-lens-clu...ng-lenses.html) and you will see every possible option that people have tried.