ZIL & L2ARC can and (carefully saying) should be added later, unless you're sure they are needed. so first, check the performance you get from the system without them, and then see if you really need them.Also, I think I should add a couple of SSD, one for the ZIL and another for the LARC, but I dunno what ssd to choose or if I boost the performance of the nas.
Also, "MIRROR the ZIL" & "STRIPE the L2ARC" - don't use one SSD for each.
Take a look at cyberjock's signature, and read his guide on that.