I brought a few 14tb seagate external drives for $199 ... schucked the drives and it is being used on my Synology 920+
Glad that works for you. However, unless things have changed in the past few years - you want NAS drives in your NAS. Non-NAS eg. internal and external drives do not flag repeated errors as soon as a NAS drive will. So when used in a NAS, they may not give early enough warning for the NAS to recover. I'm no expert but lots has been written on that.
These Reds on sale do look great for a home NAS. I love my Synology dual-bay which with two WD reds has run flawlessly 24/7 for 4+ years. Compact, almost zero setup or knowledge required, self-updating, and easily worth the cost over making something out of an old PC. Just my two cents.