If you insist on building your own unit, get a 2” Wilden air-operated, dual-diaphragm pump. However, you will most likely spend the same amount (plus your time and effort) in building your own unit versus just buying a professional seal coating unit. I mean by building a real, professional unit with a round tank and hydraulic agitation, not one of those plastic totes with a trash pump setups people on this forum seem to worship. EVERY high-quality sealer requires some sand. Even stuff at home depot sold in buckets has some sand. The pump on those plastic tote setups are cheap trash pumps designed for pumping water from flooded basements and things like that. They are not designed to transfer and apply the heavy sand loads a professional sealer mix should contain.