Couple things I would like to add...
First I spoke with one of the engineers at work, I have heard many people say that on a closed loop, the gravity on the suction side of the pump will equalize the head pressure the pump must push against. That is incorrect, true for every28"(?) water will have 1psi of pressure, but you must consider that your pump is pushing water up faster than what gravity will it down. So you must still consider how high the pump will push.
Second I was about to buy a sequence pump from AZponds, but first I called Sequence Pumps directly and spoke with them. Yes the pumps from AZ ponds can come with the Salt Water seal, however it does not come with the salt water shaft (pvc coated vs. steel), or the salt water hardware (higher grade of stainless) Sequence straight out said, "I would not buy a pump for a salt water application from a pond supply store".
FWIW