For KSP, I would wait until AMD releases their new Ryzen processors, they are rumored to be better than (or equal to) Intel's at a lower price point. The high-end Ryzens (~$400 from rumor) are coming in January and the lower end ones in late Q1 or so from rumors. Processor is more important in KSP, I'm not really sure what goes into a processor besides core count and clock speed (I know there's more, but not really what it does, that's why an i5 6600k is better than a FX 8350). A dedicated GPU would make a huge difference for games and visuals in KSP, if you're on a budget a RX 460 2GB is a great card at just under $100 for one. If he does other games and would like to make use of the FreeSync support technology in a Radeon graphics card and you can spare some more money a FreeSync supported monitor would be a good idea to purchase. FreeSync matches the refresh rate of the monitor to the framerate in-game if it is in the range (Range for the linked monitor is 30Hz-75Hz I believe for this monitor). It's a luxury, so you don't need it, but if you have a supported Radeon card and are looking for a new monitor that's a good monitor to pair up with a RX 460. For general gaming right now this should do well for a low price (includes a wireless network card, 120GB solid state drive for faster boot times and loading KSP a lot faster, and the monitor): https://pcpartpicker.com/list/rrfxTH
I assume you already have a monitor, so if you don't need that (it's more of a luxury), here's a list without it: https://pcpartpicker.com/list/K8wm2R
You'll still need a Windows key for both, but you can use the Windows product key from your old computer if you can find it and register with that to save the $100 on a key. If you need an optical drive they are about $15, but you can use a flash drive to install Windows and that will work perfectly fine. And finally, the motherboard in the list supports 4 slots of RAM, and I chose a 2x4 set of RAM, which when installed will allow higher bandwidth because it is two sticks of RAM instead of one, and because it is 4 slots of RAM you can upgrade more later. I didn't include a hard drive either because you can wipe your old one and use that for other storage and keep the SSD for Windows, KSP, and a few other things.
I feel I left some stuff out.... I'll edit post with a different list if people on the r/pcmasterrace Discord have suggestions to make it better for the price.