[muzzza]

Craig, Leaving a gap will invite snakes, mice, geckos etc. I would not be leaving a gap at all. Best practice is to lap the cladding down the side of the slab min 50mm. Between slab edge and the cladding, I normally specify a "Z" type flashing that runs the perimeter of the slab. This flashing closes off the corrugated ribs in the wall cladding to make it pretty much vermin proof.
Sloping slab will be a pain the arse in every way, other than when you are hosing ! ( hosing is for cowsheds !! IMHO ) Can't imagine you will get that much crap on the floor !?. Wash down bay with catch drains outside entry doors would be my idea. Interesting that you have embedded steel portal frames directly into bored concrete piers. Normally we would set bolts into reinforced pier to enable adjustments / levelling.
Trust that a good bitumen based paint was used on steelwork in contact with concrete. Muz