The reason I say this, is so you can be involved and take your time figuring out your best configuration. You will have experience mining, and then pick up some things over time chatting with people. Then, you can make your perfect miner for you, and also have this first miner working as well.
Here are some more answers -
You need an sas2 controller. sas1 only supports 2TB.
Mining is not intensive, though I don’t have any experience personally mining with gpu. The CPU will be run on average every 4 minutes to scan the drives, and the other time it will be idle. If you are just scanning a couple drives with the cpu, there isn’t much going on there at all…however if you are scanning many many drives, it will be taking a lot of the cpu time.
Motherboards…I don’t have any specific recommendation as I have just used what is lying around. I prefer CPU mining as I didn’t have good luck when I tried GPU way back when I started. So, if I were buying a motherboard, I would probably want a multi-socket that supports CPUs with as many threads as possible (this would be for the sas configuration).
As for WD Green…any drives mine well. People even use the cheapest seagate external smr drives. I do know some people go after enterprise drives because of the good warranty.
What I’m trying to say here is this: Try to go the cheapest way in the beginning. You will gather more knowledge and experience and will be able to better determine what your perfect miner is that you want to make. Much of a miner build is according to personal preference, not that there is a “perfect” build.
Here is a plotting Benchmark page maintained by @MrPilotMan as well. You might find this useful https://www.ecomine.earth/plottingbenchmarks/