Low budget with room for upgrades.

Hello all! I haven’t visited the forum for many years, but I had an idea that the forum and buying advice section might still be there, so that’s what happened 👍🏼

Helping a friend find/build a computer for her son. I built my last PC in 2012 and a lot has happened since the Sandy/Ivy time that I’ve noticed 😁.
I haven’t kept up with developments, so I try to read as much as I can.
The basic idea in the build is that it will be possible to upgrade the gpu in the future but keep the cost low for now (solo with multiple kids) and maybe find a Gtx1080 or similar to start with (suggestions?) (presumably fortnite, cs2 1080/ 1440p).
The boy mostly plays Fortnite now on PS but I can imagine as he gets older he wants to test other fps etc in the future and that you then go with a heavier GPU.

To keep the cost down, I thought about building on top of an older Coolermaster HAF X chassis I have (just thought about adding some new RGB fans).

I did a little digging for other buying tips and found this recipe, which seems to be a perfectly good base that will last a while.

https://www.inet.se/kundvagn/visa/oQUeAYX09JD9DztNlhBt-5cCrp8/rig

Can the cost be further reduced by using one of my old PSUs if they are compatible? Corsair Ax850 Gold?

Feel free to come up with solutions/suggestions/objections or other questions. I expect the budget to be 6-7 thousand

Many thanks to everyone who takes the time to help 🙂