Asus Radeon R7 370 - System Build - PCPartPicker
These would be my choices given your situation, picked the motherboard as it not only includes an M.2 slot as an option, it also has good overclocking capabilities and 4 SATA3 ports.
RAM is basically get whatever is cheapest, as proper reliability only really comes with ECC RAM which is for servers.
CPU Cooler was based on both performance and reliability, liquid cooling would be something of an obvious go-to but liquid coolers have more points of failure and that's not something you want on a system that holds a relatively significant amount of data.
HDD's I assumed you meant 4TB as opposed to 4GB.
The GPU was picked namely for the price to performance as you specified.
PSU was picked again for reliability, not to mention using the system as a NAS generally means constant uptime, so I would consider efficiency a factor (hence the Gold certification).