Read these two books first.
(1) Investing in Real Estate, 4th edition or later, by Andrew McLean & Gary W. Eldred (who's taken over the new editions) and
(2) Landlording by Leigh Robinson (7th edition or later).
They're pretty widely available and could be at your local library.
Most rental units run at a 4-6% return on invested cash. If you're planning to use a manager that will eat up the cashflow. The Eldred book will help you ensure that your projections include all the real estate costs. Or, if you want an extremely realistic check on your estimates, take them to your local bank and ask for a 90% loan on your rental property. They'll tell you in 20 minutes if they think you have a good investment.
Over many decades, real estate has appreciated at approximately the rate of inflation. If you're in an area that has seen several years of excessive appreciation-- maybe even a bubble-- then your investment plan needs to consider your position if your properties revert to the mean for the next decade by dropping in value at 5% per year.
If you're missing anything, it's an appreciation of the cold fact that real estate is both an investment and a job. You get out of it the amount of hard work that you put into it. If you don't acquire the temperament to be an active manager, then hiring a property manager is like hiring a broker to buy load mutual funds. You might still make money but you'll be paying other people to do most of the work for most of your profits. And if you lose money, you'll still have to pay them.
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