Lots of videos on youtube that can help you build a PC. Check out this guy channel, he builds PC's all the time. https://www.youtube.com/user/CareyHolzman/videos
You can save money by creating your own Windows 10 Pro 64-bit USB boot drive by using window media creation tool. I recommend buying a 32GB USB 3.0 flash drive do it with. That way you can add a Setup folder that contains all of your motherboard drivers setup files you need after formatting. You can buy Windows 10 Pro 64-bit cdkey on ebay or other sites that sell cdeys. I did that back in 2017, no issues at all with my cdkey.
One good thing with pre built systems, you don't have to trouble shoot it yourself. Some companies that sell pre-built systems only give you a 1 year warranty on the parts.