ToTouchAnEmu

cat 6 will do 10gb fine up to about 30m.
 
1gbe is fine for most home nas uses, you can always upgrade to 10gbe later.
 
A old pc will work fine, you really don't need much in terms of specs, id just get what is lower power and cheap.

Yea I have made a few nas boxes and managed a few servers before.
 
Prebuilt vs diy really depends on what your goals are. DIY is more flexable, often a better value, but requires more tweaking with less support.
Id go diy if you want to tinker, go prebuilt, if you just wanna put drives in a box and be done. I have used both and both have their place.

You can raid 0 those 5tb drives to get 10tb usable, but id just get a 10tb external
 
You don't want partitions, you want image files here nomrally. much easier to manage.
Id just get a pair of these
https://www.amazon.com/Seagate-Desktop-External-Drive-STGY8000400/dp/B07CQJBSQL/ref=sr_1_3?keywords=8tb+external&qid=1557782716&s=electronics&sr=1-3
 
 
 

I personally use dd on linux for imaging, but there are simmilar tools like hdd raw copy. You will need 9tb of free space to do this though.
 
 

Intel raid normally will move to other intel boards without a data loss, but if the data is very important id make a full image of everydisk first.
 
Do you have any other systems to test with?