The case has a width of 200mm and I am asumming 20mm is for the cable routing so you will have around 170mm - 180mm of cooler clearance. The Hyper 212 EVO should fit fine since its around 163mm with the copper heatpipes.
Running in single Channel should be fine. It leaves upgradability open if you ever wanted 16GB. If you are primarily gaming it will be fine, but in tasks such as video editing you might get a bit more performance out of dual channel.
It’s a cheap ass PC. Single Channel is fine…OEMs do this all the time on budget products.
8GB is way too low. I boot into my OS and 3.6GB are gone. 8GB is 2015. Never go cheap on the RAM. 16-32GB is reasonable and you can go 2x8GB to get dual channel 
If expectations are really that low…consider a Mini PC. Less power, small, quiet and plenty for your needs then. Full PC with Tower and such only necessary with higher performance, hardware does “standard PC” quality on the phone level these days.