Depends on your needs. I don’t run intensive VMs, but I really need storage, so I went with 10x 2TB 7200 RPM drives in RAID-Z2. I can get away with it. Also, you have 8 drives, so I suggest you just go with stripped mirrors, just because it’s less of a hassle and gives you more performance (which you might need, considering you got 5200 RPM drives).
Check out Sarge’s quote from my OP here on why (in default ZFS config) you should have a certain number of drives.
Of course, you can change the default block size, but I’d personally wouldn’t bother, just go RAID10.
Depends on what cache you want. SLOG (write cache) or L2ARC (read cache). You could go with both, but I’d suggest you have SSDs in RAID1 for SLOG.
If I use a NAS, I go with NFS, because it’s easier. Also, I like qcow2. In its current state, my home lab has local storage, so I’m using ZFS directly instead of qcow2 (I’m using Proxmox) and I have a 2nd server which has a 4x 2TB drive array in RAID10 that gets ZFS snapshots for replication.
Good choice.
SLOG and L2ARC. Again, I suggest RAID1 for SLOG and maybe for L2ARC as well. Don’t have these 2 on the same mirror, use separate mirrors.