Around here in Texas all of the gated lawns you have to have a 32in, a 36in is to big for the gates. Could be different there. I would mostly focus on a walk behind as big as you can work in your area, because it will save you a ton in time. I personally would look at a belt drive walkbehind verses a hydro if your main focus is residential.
I have a bunch of residentials and we use primarly 48in and 52in cuts. If I were you though, I would go with a 36 becuase 48 and 52 inch is just to big with yards and gates. We work with $300k+ houses for the most part and won't touch a yard for under $40 and most of those yards are farily good size and don't have gates, the ones that do have gates all have a double gates.
for most metal gates, almost like chicken wire you can take them apart. then you can fit the biggest mower you have for your budget back there. always get bobcat mowers, best machine out there in my mind.
Ive been in the lawn care business for 10 years started out with 1 40" gravley. I fing that the 36" mower is ideal for gates. However theres always that one gate you will need a 21" for. We no longer take lawns with gates. They usually are not very profitable, but if your just getting started not a bad way to go.
Myself working in Fla. also, a 36" is what you want...every once in a blue moon, you'll come across a gate that you can only fit your 20" through...as for a wb, don't go smaller than a 36"...they cost more and the production ratio is not that much better than a 20" to justify the cost...you'll need the 36" to cut areas that a larger wb cannot anyway, so it's a good foundation mower...later on as you grow, purchase a larger mower to increase production...