What I know: https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm
No need to do hardware raid, mdadm is great.
I got an HBA card off of art of server on eBay, and have ungodly amounts of disk. Also, am ungodly power bill...
You can stick regular SATA drives into a SAS Bay, but not SAS drives into a SATA bay.
Some HP equipment is bitchy about non -HP drives, cards, etc.
I saw a fair amount of "Do RAID 6!" But I found on my hardware that RAID 5 and a hot standby was moderately faster.
Try not to mix drive sizes, it messes things up and wastes space.
Have fun!