No primer needed. If you do a good prep to get to bare metal an exterior-grade latex gloss will hold up just fine and save a money too. Use a latex floor enamel for extra toughness.

I'm convinced that when it comes to paint there is usually no need to pay the "boat tax". There's a lot more R&D $$ spent by paint companies on higher demand markets (like automotive and buildings) where there is also more price competition.