Personally I would avoid doing a self-build, purely only because if you're dropping around a grand and you're unfamiliar with troubleshooting and the kinds of issues that can occur, it can make for a miserable experience dealing with RMA's and the like, if you run into problems or (heaven forbid) you break a component. I've been there in all scenarios and it's a real buzzkill.
That said it'd be a great father / son type exercise of course and there's never been a better time in terms of quality of components and the wealth of knowledge out there. Linus Tech Tips did a really good, super
comprehensive video on the process.. probably the only video / guide you'll need.
https://www.youtube.com/watch?v=BL4DCEp7blY
On the subject of monitors, honestly I'm going to go against the grain here somewhat, but ultimately it's entirely your money and decision... don't spend loads of money on the monitor. You can get great quality monitors for peanuts and even if he's a gamer, I just don't understand the need for 144hz (or higher) - I game at 60hz on my modest rig and when I'm on form, I'm very competitive on the online games I play (e.g. Battlefield) - I'm not sure that a faster refresh rate monitor would make any difference... I'd rather put the money into better CPU/graphics card/memory.