The NH-D15 would be fine with the 8700k. It's a pretty ugly cooler though IMO and I'd recommend something nicer, like a Be Quiet! Dark Rock Pro 4 or Phanteks PH-TC14PE. Cryorig R1 is another decent option.

I wouldn't buy a 750D. It's a pretty unnecessarily large case and old at this point. Buy something newer for sure, you can get some really good cases in the $70-100 price range these days, and there's a good number of decent options in the $150 range. Fractal Design Define R6, Define S2, Meshify C, Phanteks Eclipse P350X, Enthoo Pro M TG, Enthoo EVOLV ATX, Enthoo Pro TG, NZXT H500, H700 are just a few choices in the $70-150 range.

For 1440p high refresh rate, I'd probably get this since it's a great price;
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The stand is pretty shitty though so I'd buy a monitor arm/stand separately for it, or you can spend a bit more for the same monitor with a better stand. If you want G-Sync(for an added cost, of course), then this is a good shout;
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or the Asus PG279Q if you don't mind spending the extra $100 for it;
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I'd stick with the Asus motherboard. Asus have the best BIOS, hands down. The higher end Strix Z370 boards would probably be fine though, no need for a Maximus series board. That said, nothing wrong with the AsRock Taichi boards. Although, Z390 motherboards are out now so I'd grab one of those instead of Z370. Also, don't buy an 8700k now, 9th gen CPUs launch in like 2 weeks which is bringing the 8C/8T i7-9700k and 8C/16T i9-9900k. For RAM, just grab the cheapest 2x8GB 2666MHz+ kit that you like the look of. No, you do not need a sound card. Z390 chipset comes with integrated Wi-Fi AFAIK so going Z390 instead of Z370 should solve that wi-fi issue, but double check the motherboard you're buying has integrated wi-fi if that's a necessity for you. Otherwise, you can buy a wireless network adapter for like $20-30 ez.

For mouse, I love my Corsair Raptor M45 so if I was buying a new mouse, I'd want to go with something similar which would be something like an M65 PRO RGB, or the Dark Core RGB for a wireless mouse. Logitech have some popular offerings, but I personally don't like most of the Logitech mice I've used. Mouse choice is going to be entirely subjective though so just figure out a rough budget and see what's available that you like the look of in that price range.

I'd probably recommend dropping that 970 EVO for an 860 EVO, it'll save you some cash and you most likely won't notice the difference since you're just gaming/streaming which isn't really going to utilise the speeds of the NVMe 970 EVO. That said, the 970 EVO is still a ridiculously good price so it wouldn't be a bad choice- you might just be better off saving $60 and getting a 500GB 860 EVO, or spending $20 more and getting twice the capacity with a 1TB 860 EVO.

PSU is fine, but keep an eye on Corsair RMx, EVGA G2/G3, and Bitfenix Whisper M 750W and 850W PSUs, they go on sale for really good prices occasionally. If you don't need/want SLI support, then a 550W PSU would be plenty, but 750-850W gives you that room for a multi-GPU set-up if you decide to do so in the future.