unless all the lawns are tiny in your area, if you grow beyond a 1 man operation you will eventually need a 60" plus zero turn and a large size w/b, plus a smaller mower for gates and what-not. since you aren't there yet, get equipment you can use both now and then. because of money, wait on the zero turn. if you have a little money, get a used 54" hydro w/b and a 21" 5+ hp push mower w/ big wheels. if you have a little more money available, get a big w/b and a small gear driven w/b, one whose outer width on the deck is less than 36". (a 36" mower has a wider than 36" deck).
just my humble lil ole opinion