Always a matter of opinion here but I only use WD mostly and some Hitachi.
I don't understand why you want to take apart the existing drives as they function just fine as they are.
You can use any hard drive inside the Nas drive units and buying 7200 Rpm drives make the most sense.
Out of curiosity are your external drives 3.5" drives or 2.5" laptop drives anyway?
Someone in here mentioned that a Nas drive is faster than a server and that is just not true at all. Best performing on a network are gigabitconnections for Nas and for your computers and while there isn't a huge difference today there will be as time goes on.
I used to use a gigabit Buffalo NAS drives for files for 4 computers, but the life expectancy isn't that long and I don't know why but I burned through them every 3 years. From the time I started using servers, I have long been away from those. I just bought a small Lenovo server with a xeon E5-2000 range cpu and outfitted it with two 256 gb ssd drives and 16 gb ram for under $500 and my files fly all over this place!
I want to take apart the existing externals to reduce clutter. That is really my only goal there. Putting them in a NAS would have done it and now, putting them in a custom tower will as well. Not sure on those measurements, but they are WD Elements, if helps.
I'm still not sure what I'll do on further expansions though. Hopefully something that keeps clutter down and doesn't require drives to be plugged into a laptop 24/7. I'd like wireless accessibility (from computer, roku, and ps4) and the option for a sleep mode and maybe self diagnostic checks.
The down side I am now seeing to NAS's (for me, anyway) is that I am forever expanding my drive sizes when I can just fill up 2 tb after 2 tb drives for cheaper than buying a 5 and then a 10 (for instance).