Six drives is a good number of physical volumes for a RAIDZ2 - just group them into one vdev, then (if you need to) split off quota'd sections using datasets. Leave the spare 1Tb drive in a locked cupboard somewhere to swap in, don't build it into the system. With RAIDZ2 you get a safe warning period, since you're still redundant when one dies, and having to manually switch a disk out makes sure that you get a new spare bought in ASAP. Also means you know which disks are active in the system - all of them! Avoids potential screwups by pulling the wrong drive later.
It doesn't matter which ports you use, just make sure that physically anything mounted on the eSATAs is safely locked away and perhaps put a blob of hotglue on the connectors if that makes sense.
Is there some reason you want to use 1Tb drives? A four 2Tb RAIDZ2 would give you the same protection for much the same price and allow you to fit all the drives (except the spare!) internally, likely run a bit faster, cooler and lower power.
Usual warnings apply about not buying all your drives identical in one box - raises the odds that they're the same production run, and have had the same environment all their "life", so that they'll start failing at much the same time.
You may find that the L2ARC isn't necessary, as your system has plenty of RAM and is likely rate limited by the network rather than the disks. Try it with and without. If your clients are Windows based, ie you're using SMB shares, be aware that Samba is single threaded and just won't be all that fast. Plus you don't have many clients.
