GPU: 1050 Ti would probably be good enough for you. Here's a review of it:
GeForce GTX 1050 Ti review | PC Gamer
It's good enough for most esports titles (CSGO, DOTA 2, Fortnite) and can play others at low to medium (PUBG, RS:S). A better option might be GTX 970, as it outperforms the GTX 1050 Ti. You can check out the site's own marketplace, there's a lot of used graphics cards going for cheap here. Remember to always search up prices and don't overpay:
https://www.pakgamers.com/forums/f64/
Motherboard: The guy is bullshitting you. Z370 boards are overclocking boards that are unneeded when you have a non-K chip. H370/B360 boards are cheaper, and should always be. H370 should be at a 12k-13k range. B360 is too barebones for you, so H370 is the go-to here.
HDD/SSD: Go for the Seagate one. There's honestly little performance difference here, so go for the cheaper option. I will definitely suggest an SSD, even if it's 128 GB because it really does make a difference when loading your OS. Windows 10 is really taxing on an HDD. In my personal experience, Windows services like Superfetch keep my HDD running at 100% for 5-10 mins after boot so that they can cache some stuff in RAM. This has essentially killed my hard drive's health. So yes, SSD for boot drive, always.
CPU: 8400 is definitely enough for your uses, no worries here. Just remember to keep it cool.
I don't know much about power surge protection, afaik good power supplies have it built in them. Definitely get others' advice on this as I can't really say.