Personally, I'd use a 3TB HDD for mass storage. They're about $100AUD and would suffice for all your documents, music, videos, as well as games and programs where speed isn't a factor. Just make sure your case has a 5.25" bay and you get a SATA data cable for it. Your PSU should come with a SATA power cable.
Didn't see the motherboard in your recent list, but an M.2 drive won't interfere with your graphics card unless the heatsink (which yours has) is too big. That drive is 8.6mm thick, which should be around the size of the socket it plugs into, which should be fine.
Edit: I'd also echo
@Skube's point on upgrading your PSU to 850W if you can swing the cost. A higher wattage PSU will future-proof against upgrades, and they only use as much power as they need anyway. If you don't intend to upgrade, then it's sufficient and has a 10 year warranty.