According to the tables published by Hypro, 2.5 HP is enough for a D252GRGI. These pumps are comparatively low maintenance and service of wear parts(diaphragms and valves) is simple. I read what needs to be done to service a roller pump, no thanks. Diaphragm pumps will also handle being run dry. Same cannot be said for roller pumps.
An air gap is a simple thing to make. Mine is nothing more than PVC elbowed so that the outlet is held 4" above the top of the tank opening. Having that makes me legal according to DOA regulations to fill almost anywhere.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.
Benjamin Franklin 1775
Sell not virtue to purchase wealth, nor Liberty to purchase power.
Benjamin Franklin Poor Richard's Almanac1738