I would generally reccommend against RAIDZ1: either go mirrors or RAIDZ2/3. Either way with the savings from MB+CPU you should have the budget for larger and/or more drives.
Also, take a look at the Toshiba MG HDD line, they are great drives and usually quite cheaper than WD or Seagate. Oh, don’t buy used drives! And avoid SMR ones as they are not compatible with ZFS.
Choosing the pool layout is the system planning part I prefer the most!
Suggested readings are iX's ZFS Pool Layout White Paper, Assessing the Potential for Data Loss | TrueNAS Community and 10 Gig Networking Primer | TrueNAS Community.