For light use like yours would be, you can save a bundle of $$$ buying a mower with some hours on it. I wouldn't be afraid of a mower with as much as 1500 hours on it. Look at it, you'll know if it was beat to death or maintained. Do an engine compression check, check the dip stick and see if it's carbonized on the bottom and that the oil isn't black sludge, look for signs of leaks (they'll usually accumulate dirt and dust around them), check the belts for nicks and cuts, look at the spindles, look for rust particularly on the deck & muffler (some rust is normal) etc... go ahead kick the tires and take it for a run.
I bought my first commercial rider (X-Mark Lazer Z 52" 25HP Kohler) with 1700 hours on it for $2,000. I saved a good $6-8,000 over a new one. That was five years ago, I use it commercially, mostly on residential properties. It's still running today and I haven't spent $4,000 - $6,000 outside of normal maintenance and wear and tear. My biggest expenses were the PTO Clutch@ $300 and a new head for the motor $750. All in all it was a good buy and it's still running.
Everyone here talks about regular maintenance, this is a must (In zone 9, I change plugs 4 x per year), Rust is a problem so cleaning the deck after you finish cutting is important, regular oil and filter changes, (we do them weekly since the machines are grinding every day, all day) make sure the zerk fittings (those little nobbies you sguirt grease into) are wiped immaculatly clean before you lube otherwise you'll jam dirt into the grease and increase wear. Oh, and don't over grease them either, a squirt ought to be enough.
There are plenty for sale, specially at the end of a growing season or the beginning, patience pays.