If you're literally just trying to play Fortnite, I would recommend looking for an older business workstation with a decent i3/i5, then purchase a graphics card for it. You can find older Dell/HP/Lenovo/etc. workstations with 3rd/4th gen i3/i5's for less than $200, and any used graphics card around the $100 mark would be sufficient, a decent choice being the GTX 750 ti.
Saves you the hassle of having to build anything.