CPU: I have no idea about that in context of a home server, but for running 14/7 I know that xeons are the best, and the upcoming epyc would compete with them. I have pentiums and celerons seen being used for that. If power consumption is one of the priorities, I would certainly look at performance per TDP. The G4600T has a TDP of 25W so a good option. The xeon E3-1275V6 is as expensive as an i7 7700K and with a TDP of 73W it seems a bit powerhunrgy for a file server and such. Than I would say to have a look at the i7 7700T which is a bit less costly, 4C/8T and has a TDP of 35W, but I don't know how great it is to run 24/7. It gives 90% of the performance of the xeon in cinebench R11,5, R15, CPU Passmark and geekbench 3 (I don't know how relevant these results are though).
ECC ram depends on how much safety you want. It can detect if there are uneven faults in the binary string (so more than half of the faults made would be recognized) but I don't know if it it matter for regular users that much. About price I know nothing, but if it is so much more than regular ram than surely recommend to asses if you have the need for it.
Motherbord: search what suits you best. I for example would always recommend one with WiFi build in, certainly when you would add a lot of PCIe cards. But for a home server I don't think there would be that many PCIe cards so I don't see an issue there. I would look for 10 gigabit ethernet if it falls in the price margin so that it is future proof (if plans are going to be to move to 10 gigabit ethernet ofcourse). Also interesting is to watch out how many SSD's/HDD's can be connected to the motherboard.
Storage: definitely get a raid setup, the safety for losing a HDD is great. Here we are just regular people with a NAS with 2 2TB HDD in it. We didn't need that much of storage, but it was on sale so the best option. Since we didn't need 4TB we decided tot put it in raid 1. Man, did that save us 2 times! We never knew we would need it, but now I can say we definitely won't go back to single HDD NAS. You have 4 cards so you have plenty of options. I personally would do a raid 10 (which is groups of 2 disk that mirror each other, and those groups than put in raid 0) it would give you an effective 6TB of storage, the other 6TB is the backup, and it gives you double the speed of what a single HDD would achieve since the information is stripped over the drives, the system can handle to lose a HDD per group, so up to 2 drives in you case if they are not the ones mirroring eachother. If you want a bit more speed, you can have a look at raid 5 or 6. Raid 5 would give you 9 TB of storage, 3 times the speed of a HDD and when 1 HDD is lost, the system can still move on with the 3TB of parity blocks (don't ask me how) so you can change the faulty HDD. Downside is that it is that it is recommended to use a hardware controller (not necessary but recommended), also it can take some time to get the data on the replacement HDD since it has to be calculated from the parity blocks. Raid 6 is also an option and looks like raid 5 in that it uses parity blocks to recalculate the data from a HDD if it went lost. But it uses 2 parity blocks divided over the drives so it can handle 2 lost drives. It would not give the same storage and speed bump as raid 5 since the extra parity blocks also take their space, but it does better than raid 10.
I would suggest raid 10 since 6TB is plenty for most people, and lossing an HDD doesn't affect the system. With raid 5 and 6 the lost data is calculated from the parity blocks, so it affects the system by slowing down the speed for data that have to be restored.
The biggest question ofcourse is how professional you want it to be. Getting a xeon for it, ECC memory and such are the professional options (and costly) that aren't really needed for a home server. They can be handy but I don't know if they are really worth the price for a simple home server. Unless you have some professional needs for it ofcourse.
If I had the money and need for a home server, I would probably use the i7 7700T since it has a TDP of 35W and has some decent performance, and get a B250 or H270 motherboard, regular ram and 4 HDD's in raid 10. For the moment there is more need for a desktop build so I won't do a home server build though.