There is also room to add on and make it a triplex, I need to get quotes for the cost. 

Looking for something that helps factor in all variables, fixed costs, interest paid on hard money loans, reno costs, etc etc. 

Reliable and Cost-Effective Drain Services in Cleveland, Ohio As landlords operating in Cleveland, Ohio, we have often faced challenges finding a cost-effective and consistent drain service provider.

I just don't feel that I'm at the point yet where the benefits justify the cost of Buildium for my portfolio. 

For us in California the main metric is not "low cost".

This AC costs $17000 in 2020 and assume will no problem for a long time but most expensive one and shortest time died. 

I've done my fair share of research into good 'rules of thumb', but not any that consider the cost of land in the area.

Are there any red flags I should be aware of, such as the vacancy expense or NNN costs?

This process can be very costly, requires zoning permission, in addition to all the items you listed.

So if you happen to have 2 of those types of things happen in any given month.. .or the thousand other things that can go sideways with rentals where you don't see your money for some reason... you are now minus $2,500 in profit for that month.