As I grew I learned some things the hard way:
#1. Owner/operators are lot better at tenant selection. I self managed until we got to 60 houses (taking on 3-4 new ones a month). After that we contracted a person that works for just us. His gut on tenants is much worse than mine. Cost of doing business and I sure appreciate not meeting tenants any more. In my experience, property manager companies are usually worse (either way too picky or way too loose).
#2. Low end houses, rent under $800, your expense ratio will be 40% minimum probably more (even if you're good). I'm in California on my $1,500/month rentals I run in the 30% range. You will beat the average when you're small, but as you grow I cannot.
#3. Financing/borrowing money is the KEY to this business especially if you want fast growth. Don't write everything you can off, banks won't lend to you. Don't buy super low-end, banks won't lend to you. Don't try to explain to banks how you buy substantially under market value, they won't believe you. Refinance those properties after they are performing. Most regional banks have a customer lending limit and some rules (I think 3% of the outstanding loans to one customer). Learn how to calculate a debt service coverage ratio and bide your time. It took me 2 years of solid taxes for banks to start lending to me. I can go on and on about this. Fast growth also scares them. In my experience, the bankers I deal with didn't like the hedge fund approach, I own 12 properties with a hedge fund and they didn't like lending to that. Banks, hard money lenders, private lenders and family and friends money all have there place.
#4. Convert your maintenance requests to an online email form. This was huge for us, it sucks to sometimes get 3-4 in one day, but they will only go to the site and type it out so I can make sure it gets done and track who is always whining.
#5. Make the cash tenants deposit rent directly to your bank. The Teller gives them a receipt and does the customer service. Make the other ones ACH you money.
#6. Have a solid long-term vision: what is the benefit of owning 500, 30k houses versus 250 60k houses? For a small market, is your concentration too high? Why not sell and buy commercial or apartment.
#7. Know where you make your money. You have picked up 366,000 in cost with a value of 605,000. The time you spend on finding GREAT deals is what is important versus trying to make your expense ratio 40% instead of 50% with brute force.
Good luck. It is cool to see so many people here are into real estate.