Raid 5, easy to set up, just follow the instructions here: interface has changed a little, but if you are used to computers, the basis is still the same.
http://rockstor.com/docs/pools-btrfs.html
For Teaming: no problem to install, just use 8-13 stable or prepare to install NetworkManager-team by CLI. Error when creating team - #3 by bug11
For install the way you describe,OS on USB, then add 2 disks, copy data over and add more disks, that should be OK. Have backup of important stuff (I take no responsibility).