I'm going to take a slightly less pessimistic point of view. At 32GB-to-3.6TB, the ratio is sufficient that I haven't seen anyone report catastrophes in that range. cyberjock is being appropriately paranoid in guiding you away from a
feature that can be playing with fire. The FreeNAS developers have seen situations where
pool import takes about 5GB RAM per TB of disk, so my guess would be that you'd be unlikely to wreck.
From a practical point of view, I seem to recall that WAFL uses fixed block sizes of 4K and that NetApp deduplication works on that basis. If that is true, it is worth considering that ZFS uses a
variable block size, and that the resulting dedup savings would be likely to be at least a bit lower.
Since you've given us nothing to work with on the horsepower side of things, I'll mostly avoid that question. Dedup reduces write performance somewhat, but the reduced number of blocks improves ARC/L2ARC performance and responsiveness, so it is a tradeoff.
Do note that filling a ZFS pool results in bad things. Normally it is advised not to exceed 80%. ZFS and iSCSI are good at causing CoW fragmentation, so for iSCSI I feel that might be more like 60% (but it is very dependent on workload etc). dedup can help create free space, but you probably shouldn't be anxious to go filling modest amounts of generated free space with other new content.