+1 on everything Adam suggested. You mention you have a "good amount" of Pentax lenses. Were they film era lenses or APS-C glass for your older K100D?
In trying to decide how to best spend $2000, the main question is how much is needed for better optics because youʻre upgrading to a better (or bigger) sensor? Upgrading your lenses (if needed) will have a much bigger impact than upgrading the camera. Therefore, depending on how much you need to budget for the lenses, that will determine if you can afford a KP or K70. (The K1 could be an option if your budget was closer to $3k).
I have the luxury of shooting both Pentax and Nikon, and can vouch that the D750 is a great camera. But it was new in 2014 and is due to be replaced by Nikon. And unless youʻre willing to either sell all your Pentax gear to switch to Nikon, or plan on periodically buying more Nikon lenses over time, staying with Pentax makes the most sense. Again, the idea is that you make long term purchases of great lenses, and then over time upgrade your camera that works with those lenses.
Thatʻs me, anyway. Most folks just buy the camera with a kit lens and get very good results for 4-6 years and then upgrade to the latest camera with a new kit lens.
And my perspective on auto focus? I learned to shoot action as a pro before AF existed with rolls of 15-36 exposures. When Minolta first came out with AF, I was ridiculed by my peers like as if I had a focusing disability. Faster and more accurate AF in 2018 is nice, but I never blame the technology for missing the shot. Whatʻs most important to me in an AF lens, is the ability to quick shift and tweak the AF manually.
---------- Post added 03-19-18 at 01:31 PM ----------
Originally posted by meeseeks84 When I picked up these lenses a while ago from a buddy that was "liquidating" his camera stuff, he had a K3-II. Would that be one to consider as well?
These are the Pentax lenses I currently have (or the ones I find I use the most):
SMC Pentax-DA 2.8 40mm Limited
SMC Pentax-FA 1.8 77mm Limited
HD Pentax-DA 4 15mm ED AL Limited
SMC Pentax-DFA Macro 2.8 100mm WR
Can you live without a built-in flash? The K70 flash is a bit stronger than the KP flash, and the K3 II has none. Thatʻs an excellent collection of primes. If I were you, Iʻd go the KP route with perhaps a 16-85mm zoom for convenience and WR.