I’m not sure how reliable CoolerMaster’s power supplies are, so I would at least consider something by Enermax, Tagan, Hiper, or Antec. Check what the 12V rail is spec’d. I believe that the CoolerMaster you’re looking at doesn’t have 20A on the 12V rail, and more likely around 18A which might work if this isn’t going to be a gaming rig.
For the higher speed RAM, yes you can use it, but unless you plan to overclock heavily, you’re going to find getting PC4000 to be a worthless investment.
The system may be fine for your cousin, but that depends on what it’s going to be used for.