Some cases are easier to work in i.e more space around the motherboard and better cable management, and have better airflow etc. You also might get better/more fans in an £80/£90 case than you would with a £50 case. You also have cases with tempered glass side panels if you want a nice big window such as the Fractal Design Meshify and Phanteks Enthoo Pro M Glass.
You can always look at some reviews to get another opinion. Websites like Kitguru, Techpowerup and Tweaktown all have reviews on pc hardware.
The Crucial MX300 525GB is pretty good value compared to the 850 EVO. You are not going to notice any difference in speed in day to day usage (boot times/loading games) plus you get 25GB extra for less money.
My basket at Overclockers UK:
Total: £158.69
(includes shipping: £8.70)
It is easier than you might think to build a pc. The key is to take your time and double check everything. Once you have the board in and cooler installed it is pretty straightforward. Then it is just a case of plugging in the other components i.e Ram, Gpu and storage etc and sorting out the cabling mess.
I would install the psu initially and feed the 8 pin cable through the bottom cable routing hole and then run it out at the top left where it plugs into the boards 8 pin connector. It can be awkward otherwise to plug in once you have the cpu cooler installed as there isn't a lot of room to work with.
It also makes life a bit easier if you download the AMD chipset and Ethernet (LAN) drivers from the motherboard website, so in your case Asus. Also download the gpu driver from Nvidia. Just put them all on a usb stick.
After you have an internet connection you can look at installing/updating any other drivers i.e Audio and anything else that might need it. You can look in device manager after installing windows to see if there are any issues. The driver support out of the box might be a lot better with Win10 than it was back in the days of Win7/Win8.
There are lots of pc build guides on youtube if you want to watch something before hand. And if you do get stuck just start a thread in general hardware and we will do our best to assist.
As for anti static wristbands I find they are more of a hinderance as they get in the way. Just touch a bare copper radiator pipe before you start.