When my buddy and I first went into business we had a 21" Snapper push mower (new), a 42" Craftsman riding mower (3 yrs. old), a 48" Bobcat walk behind (18 yrs. old with a recently replaced engine), 2 Craftsman weed eaters, and a 6' x 12' trailer. After the first year, we took advantage of a finance program Toro was running and bought a 48" walk behind with sulky and a 52" home owner grade zero turn. We soon realized buying the 52" home owner grade zero was a big mistake. We sold it and bought a used (550 hrs.) 60" Exmark Lazer Z and a used (450 hrs.) 60" Toro Z Master. When we bought those commercial grade 60" zeros, we really started making a living mowing grass.
My advice would be to buy a used 48" walk behind (0 - 800 hrs.) and possibly add a sulky. If at all possible, add a 60" commercial zero turn (0 - 800 hrs.). You can use a push mower for any small areas where the 48" wont go. The thing I would stress the most is to buy commercial grade. I would highly recommend Exmark, Toro, and Scag. These brands are tried and true and if something should go wrong, they are easy to get parts for. I'd pass on all others, especially John Deere. I apologize to any Deere fans reading this but if you gave me one, I'd sell it the next day.