RAID 1 is not the only backup contingent to be sure. I suggested it in case of actual hardware failure. ie the OS drive craps out. Then he could just reset bios and quickly access and continue to work. Point being was to provide a contingency plan for the just in cases. A kid in school may be having exams tomorrow, if the OS HDD hardware breaks tonight, he would have minimal downtime and can keep working. Fixing overall system when time permits.
I myself would get a personal cloud HDD. Separate, easially unplugable, convienant, close at hand when you want to backup certain files as well as OS drive, more cost. Western Digital, 1 TB cloud, $130.00
I myself would never use a super large drive for OS drive, but that's me.
I use a 900w psu in my game computer, listed in sig. It is true that modern components use less power, multi vid card systems need a little something something tho. If system does not need it, it will not hurt anything but wallet. System will draw whats necessary and that's it. Cannot hurt. Just makes for beefier PSU guts. Afterall its about the amps and heat. Certain things can be used over long periods of time if they are quality, a PSU is one of these. Like a good case, it can be used for new builds in future. I fully expect to never buy a new case or PSU again, I bought quality now for life time service, my life time that is.
You never know, he might let a friend convince him to push the system, they are kids. Plus you gotta know he will play games on it.