The current version of the pump you have is most likely: http://www.campingworld.com/browse/s...3&skunum=20066
They are inexpensive, reliable, noisy, offer direct replacement, and fairly low pressure. This is not altogether a bad thing as, if I recall, you still have the original polybutylene plumbing.
The variable speed pumps are a nice upgrade, but the chief benefit comes if there are two people using the trailer, ie someone at the sink when you are in the shower. Oh, and they are a lot quieter, as well.
The higher capacity pumps also can pull a lot of amps. Probably more than your wiring can stand. To use one you need a smaller guage wire and probably a solenoid control from the switches. The Aquajet ES is the only one I know of that can use the existing wiring.
I personally would not buy a used pump unless it was salvage from a real late model RV. And maybe not even then. Who knows where it has been or how it has been used?