While the previous answers have given you the basic information you need, I would suggest you carefully look at the properties you want to buy and the markets where they are located. There are many markets in which $20,000 will buy a perfectly respectable property which would make an excellent rental. The Battle Creek, Michigan area is one such locale.
If you are looking to spend more, you might want to consider hiring someone as a property manager. You could purchase a repossessed property for well below market value and invest in repairs and improvements to bring top dollar for rent.
For example, there was a bank owned property in our area that was listed by the bank for $22,900. The property needed some cleaning, a little bit of flooring replaced, updated light fixtures, and a new door knob. Had this been an owner-occupied home I would have listed it for around $85,000 with the cleanup having been done and the door knob replaced. For under $2,000 all the necessary repairs could have been made. A cash offer of $20,000 likely would have bought the home.
Feel free to contact me if you would like more information about the incredible values available in my market area.