You don't 'need' to buy the RM750i - it's a great power supply, and the one I bought when I did my build - but you can choose whichever power supply you want, pretty much 99% will fit.
Be aware that most power supplies are most efficient at about 50% load, and also the more expensive power supplies are able to provide power to the system without much effort... so the fan won't even spin, giving a quieter build.
Also, there isn't a huge amount of room for cable management, so you probably want a fully modular power supply, so you don't need to find space for unnecessary cables.
I heard something the other day that your power supply should be around 10% of your total build cost. If you're putting an Intel 7700k and a GTX 1070/1080 in there, then I would say the RM750i is about right.
