1. 200 quid for a 2700x is a pretty good deal to be fair. Worth remembering that Zen 2 launches in less than a month now, and we know that the R5 3600(x) should cost around £200 but single core performance will be vastly improved over a 2700x thanks to the "15%" IPC increase, and the clock speed bump (looks like 4.6/4.7GHz should be fairly achievable across the entire Zen2 line-up). So, in games, the 3600(x) is going to be noticeably better at 1080p/1440p high refresh rate(like your 1440p 165Hz monitor). At 4k and/or 60Hz, you won't notice much difference in games, if any. In Photoshop, you're not going to notice a difference but if you're editing/rendering large/high res video's fairly often, the 2700x could be a better option thanks to the extra cores/threads.
2. You will definitely be able to use your RAM. I can't actually remember if I bought the 3000/3200Mhz kit, but I have the same RAM- pretty sure it's the 3000MHz kit, and I run it at 2933MHz A-XMP no problems. So I doubt you'll have any problems running the RAM at 2933MHz at least.
3. The PSU is fine. Technically, you could get away with a good quality 450W unit, though I definitely wouldn't with a 1080Ti. 550-650W is perfectly adequate for the vast majority of single GPU systems. You'll only really need more than 650W if you're running an HEDT CPU and/or multi-GPU system.
Absolutely do not pay that price for the 970 EVO. You can get a decent 1TB SATA SSD for that price. Or, alternatively, the 500GB 970 EVO Plus is only a few quid more on Amazon;
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I also wouldn't buy the H500i, there are better cases for that price IMO. For £70ish, the normal NZXT H500 is fine, but when you increase the cost of the case by almost 50% for some LEDs, it becomes piss poor value.
For GPU, £500 is a bit on the high end for a used 1080Ti at this point IMO, still a decent deal over an RTX 2080 though. RTX 2070Ti rumours were kicking about recently, could have been a good option instead of a used 1080Ti to be fair, but Navi pricing is such a disappointment IMO than NVidia might just not do anything to counter Navi, since they don't really need to. Still, NVidia are revealing something "Super" soon, so might be worth waiting to see? There are more benefits to the newer GPUs than just RTX cores for ray-tracing.
Oh, and for adding RAM, AMD CPU's do better with only 2 DIMMs. So if you're able to run your RAM @3200MHz A-XMP/DOCP, you might find that you have to drop frequency when you add another 2x8GB, so you might have to drop to 2933MHz(or lower) for stability with 4 DIMMs. Obviously, if you're going to benefit from the extra RAM, it doesn't matter so much but it is worth noting. If 16GB is sufficient for you, then I'd just keep it at that until you actually do need more.