Nothing. This entire rig is headed to the scap heap.
eBay the various components, or keep them stored in case you want to build a retro build for retro games. DDR3 memory still goes for a pretty penny on eBay (I tried to pick up some 4GB DDR3L 1600mhz sticks for my antiquated PC, and they were in the £20--£30 range!).
A boot drive for Windows... I'd suggest at least 240GB for the install and updates and such. Anything more would be overkill. m.2 would be the best to go, I think, but you'd have to make sure your chosen motherboard supports the right m.2. format.
I'd also suggest at least one ordinary SSD for general storage, and one traditional HDD for games and the like (they last
ages if you have a decent manufacturer's product), but game load-times et cetera might be a bit diminished compared to an SSD... but they last a
lot longer. I'm not sure you'd notice a difference between an SSD game-install and a HDD, to be honest.
Make sure whatever SSD you buy has a controller on-board -- an
example would be Crucial's MX 500 over their BX line. Without an on-board controller, an SSD's life-span is
greatly diminished.
Don't bother with a Seagate brand: their products, or at least their mechanical HDD's, are terrible and
literally wear themselves into pieces. Seriously, someone opened up their drive and it had cut itself in two!
Western Digital Black is their premier line meant for gamers/professionals.
Make sure a motherboard/case has USB 3.0/3.1 -- anything else, like SD card-readers can be purchased as USB attachments, to be used when needed.
Edit: to echo
@Brutus, go Samsung if possible, WD as a second choice from me.