Welcome brezzio {{gwi:998412}}
"ebb & flow" works on a "flood" followed by a "drain" cycle. So, I don't know how you plan on doing that with a raised supply tank.
Perhaps its easier to cut the power of the pump by installing a short circuit or a sideflow (T-piece) with a regulating tap.
By all means, clean and sanitize the pump, but perhaps a technical service might also be necessary. Trouble with an "ebb & flow" system is that the pump is the most important item. Without it there's no "ebb & flow".

you can get hydro sized pumps for 15-30 USD. go buy one better suited for hydro. unless you're going to build a monster system, you're just making more hassle than its worth.

Do not use the sump pump; you will be trying to kill a gnat with an atom bomb and wasting elec.

Sump pumps will pump upward of 40 gpm at 30' of head. What you want if you are growing 10 plants is 3 gpm at 4' of head.