I agree with the items @David Liu mentioned but add insurance, vacancy, property management (even if self managing because it is work), misc (mostly items divided across all units (bookkeeper, LLC, umbrella coverage, tax man, office expenses, etc) as well as one off items such as utilities due to slab leak.  

With $600k, I'd be more interested in gas and oil mineral rights and/or a syndication.

About 9 acres land is available .How difficult is it to get it rezoned to commercial to set up a gas station?

I have a property that I paid $140k for and had a water line leak issue that cost me, it wasn't tenant caused but still will dent my returns.

going to depend on the season and if your utilities are electric or gas . also will depend on the energy efficiency of the home 

Has the roof had any leaks?

I have a property that I'm renting where the gas and water utilities can't be split out.

I want to place wifi water sensors in all the major potential leak areas around the house.

It's four hours of windshield time and $15 in gas (minimum) every time you drive over.

If the answer is not a resounding yes I take a minute and re-evaluate what I am doing.Once you have for some people 7 figure wealth and others 8 figure wealth you let off the gas and start enjoying more fruits of your labor as you get older and time becomes more and more precious. ( Dust in the wind ) song is very real (not another minute will your dollars buy).