Yes and no. FreeNAS only needs 1GB per TB (or thereabouts) if you plan to do de-duplication (as a rule of thumb, in order to hold the ZFS de-dupe database in RAM - otherwise performance tanks horrifically).
But unless you have a very specific use case and have measured that de-duplication is a win, de-duplication isn’t worth it for most people anyway - so don’t do that, just use ZFS filesystem compression instead, which does not have that requirement.
I’d take your VM memory requirement, add 8 GB to it, and that would be a starting point for your memory requirement.
If its a single user NAS… minus the VMs and streaming, i’ve been happily running an HP Micrososerver with AMD Turion based CPU since 2012 or something and it is still running just fine. I’ve even run a few light VMs on it, but it only has 10 GB of RAM so there’s that.
But for a single user (or household) on gigabit ethernet, it is plenty.
Unless you’re running 10 GbE, going for some massively overpowered CPU, disk subsystem or massive amounts of RAM is just simply overkill.
AS to my disk config: i’m running 4 drives with a pair of mirrors.
Why?
Because i have 4 drive bays, and a pair of mirrors gives you more flexibility in upgrades. You can replace one mirror at a time and ZFS will load balance across them. I.e., to expand storage, i can do it 2 drives at a time, instead of needing to replace them all in one hit.
Yes, RAIDZ2 is in theory more reliable, but it is much slower on writes (as you only have 1 VDEV, your IOPs are limited to the speed of a single disk rather than 2 or 3 in the case of a 2x2 or 3x2 drive mirror - which will help for VMs) and disk capacity is a lot cheaper than IOPs (i.e., sure you burn 50% of your capacity for mirrors, but you might be OK with that depending on your use case and thinking forward to upgrade path down the line).
Personal opinion, for home use with small numbers of drives, 2 drive mirrors are the way to go. YMMV but just be aware that upgrading a RAIDZ2 array will hurt the hip pocket in a big way in one hit.
Hypothetically, if i have one of the drives in my box fail, i can simply either buy one replacement or buy 2 drives that are larger and get a storage capacity upgrade for minimal extra outlay…