Next: The money. The rule for investors looking to purchase for rent. I don't think it translated as well as my next rule because it is dependent on factors such as interest rates, taxes, HOA fees and such but the rule is that rent should be 10% of you purchase price. So if houses of similar size rent in your area for $1500/mo, you should plan on spending $150,000 or less on the house.
To me, a much better rule is the 3x your mortgage rule. You should look at you current total mortgage, multiply it by 3 and if comps in your area do not rent for that, you will not earn money renting your house. None of my houses rent for 3x the mortgage, hence I am losing money annually. But, I am building wealth. If you plan is to build wealth, not income, you can relax this rule somewhat but I would still stay above 2x mortgage payment, and even that is scary.
Also, do not embark on this unless you have cash reserves to cover several months without rent, major repairs and tenant turnover costs.
I am not saying this to scare you. You just need to be prepared. It is hard to make an income from rentals unless you scales but starting out small, it is still a good way to build wealth. My current real estate equity is between $1-1.5M.