With Freenas you will be better off with more smaller drives than a couple bigger drives with plans to add more later. Much easier to increase the size of the pool by swapping bigger drives (in a set) than adding more drives and combining vdevs (at least in my opinion).
Also make absolutely certain that you get CMR or PMR drives, SMR are BAD for Freenas https://www.servethehome.com/wd-red-smr-vs-cmr-tested-avoid-red-smr/
CMR=PMR https://en.wikipedia.org/wiki/Perpendicular_recording