The poster's question of a new KP vs. a used D800 was a little surprising to me. I don't have a KP, but I have two Pentax K-3 II cameras and also a Nikon D800 camera that I bought used off eBay from Japan. My D800 came to me in mint condition with only about 1400 shutter releases and for about USD $750 (not including tax but with free shipping). I've been a Pentax user for about 40 years and a Nikon F4 film shooter for about 20 years. My Nikon lens collection is small, and I've not acquired another Nikon lens in a couple of decades, nor do I ever plan to. I bought the D800 to augment my teaching videos for YouTube primarily, using my existing lens equipment, and to experiment some with 36-Mpx images in my plans to buy a K-1 II. From somebody who uses primarily Pentax cameras, I find the layout of the D800 to be frustrating and not as intuitive (despite my long owning an F4). The D800 is a large and heavy camera, much more so than even my K-3 II cameras (they are also heavy but smaller). My D800 is spending most of its working life on a tripod, whether video or still photography. For walking around and hiking hand-held photography, it'll always be Pentax for me. I guess my question is: "why new KP vs. used D800" because I would go for a mint, low-shutter-count K-3 II over a KP anyday, as it is superior in many ways, and the K-3 II is a much more apt comparison to the D800. I went from a K10D to a K-3 II, and liked the latter so much that I bought a second one as a back-up. Like the K-3 II, the D800 has two storage-card slots -- but instead of two SD card slots on the K-3 II, the D800 has one for the three types of standard UHS-1 SD cards plus the much-larger type-I CompactFlash card slot for much larger file storage, which can be a huge plus for video. The battery life on the K-3 II and D800 is much better than on the KP. And I must say that I find generally that the automatic metering in my D800 gives consistently better exposures than do my Pentax DSLRs, so there's generally less need to bracket exposures (or change EV compensation), in my experience, when I really "need" to get this particular photo or that.
As for the D800, yes, the 36-Mpx sensor is excellent. Is it that much better than a 24-Mpx sensor on a Pentax? (I'm not considering the K-1 II here, as it wasn't posted in the original question, presumably because of cost.) It depends on what you're going to do with the camera, but for most people I'd think not. As for buffering and download times, I have zero problems with my D800 shooting RAW files, but I notice that on my iMac computer, it takes a bit longer to open the larger D800 files than my K-3 II RAW files. You should have a top-notch computer with lots of memory and storage space for dealing with 36-Mpx files. When I do long astrophotos, my D800 is much quicker at buffering the photos to the card than are my Pentax DSLRs. Funnily enough, I find that the autofocus on my D800 with a Nikkor AF 35-70mm f/2.8 lens is not really better than using, say, my DA 20-40mm f/2.8-4 lens on my K-3 II. With two Nikon autofocus SLR cameras and four autofocus Pentax SLR cameras, I would say that Nikon is over-rated in autofocus and Pentax is under-rated. The continuous shutter speed is slow (3 fps) on the D800, due to the combination of it having the larger processor with the larger sensor and to the 2012 technology -- for people who care about such things (I don't). One thing I found surprising, and that it has in common with the KP (but not with the K-3 II), is that the D800 has a built-in flash; the D800 has no built-in GPS but has a port for an optional GPS.
And with my D800, I get frustrated in that the menus and buttons are often two steps (or more) vs. one step in my K-3 II. There's no hyper-manual option in my D800, for example. And the D800 doesn't have a convenient way to quickly access custom-user settings (like the U1, U2, U3 wheel settings on m K-3 II). The Nikon lenses have frustrating quirks that my Pentax lenses don't have; for example, the Nikkor AF 35-70mm f/2.8 lens has a finicky aperture-lock switch that you set with the aperture ring at f/22 (no "A" setting that is so much more practical in Pentax lenses), and I can't use the lens for aperture priority or shutter priority without that switch being set (and it's really hard to set). For me, I'd never consider getting the D800 if I didn't already have the F4 and my small Nikkor lens collection; but it's a useful camera to me in specific ways, and it is built like a tank (as is my F4, and numerous Pentax cameras too). I strongly considered the Nikon D600, D610, and D750 cameras before getting the D800, and my elimination of the first three models was because of the huge, pervasive, well-known quality-control issues that plague Nikon's manufacturing of cameras (oil droplets on sensor; focusing-defect issues, etc.); beware that the D800 is also known to have the left-side focusing problem, and Nikon has just stopped their free fixing of the problem. Final advice: if you don't already have Nikon lenses, don't start buying Nikon cameras; Pentax is much better as well as cheaper, and there are great older (film-era), used Pentax lenses of all kinds that can be used just fine on today's Pentax DSLRs (so the argument that Nikon has more and better lenses to choose from is not a proper argument). If you're looking for better video than you can get with a Pentax camera, buy a video camera, not a new DSLR system with video built in.
The original poster can also check this out:
https://cameradecision.com/compare/Pentax-KP-vs-Nikon-D800