Newbie real estate investor, what am I doing wrong?

[Max Lala]

I recently purchased a rental investment property in Dearborn Michigan for $70k and rented it for $900/month.

If I had gotten a 30 years loan on this property with %20 down, it would give me $250/month cash flow. (after taxes, insurance, estimated maintenance and management fees).

I have about $450K cash. Now the question is if I repeated this same process with 20 rental properties, it would cost me on average $17k per property (%20 downpayment including closing costs) $350K. $250/month * 20 = $5000/month, $60K year. I'd keep the other $100K for emergencies. 

$60k/year is enough for me to retire and this would be a %17 return on my $350K investment. Is this a good idea? Is there something I'm not thinking about? If you think this is a good idea, should I continue buying in the same area, or diversify with other states?

Thank you