First rental duplex: 20-year cash flow with vacancies, reserves, capex, taxes.

[Steven17]

I'm analyzing a potential duplex purchase as my first rental property, and while the numbers seem promising on the surface, I'm trying to build a thorough cash flow analysis that accounts for realistic expenses beyond just the mortgage, like vacancy rates, maintenance reserves, and potential property management fees. I'm using a standard spreadsheet, but I'm unsure about projecting accurate figures for capex and repairs for a 20-year-old building in this specific market. For experienced landlords, what are the most commonly overlooked or underestimated expenses in your first-year analysis? How do you adjust your projected cash flow for economic variables like rising property taxes or insurance costs, and what annual cash-on-cash return do you personally target to consider a property a worthwhile investment?