@Nelson Lin as others have stated, 80/20 is a good split in a syndicated model. 

Town homes are ok as long as the HOA allows rentals and <$200/mo.Mobile homes don't appreciate.Buy 3/2 houses that will rent after you move.Read the Bullet Proof Rental portfolio paper linked in the fist paragraph of my profile for a business model.

I prefer the buy low, rent high model, so yea, I buy in low income neighborhoods.  

There are some sponsorship groups where this is their entire business model...the student army. 

Go with the worst case scenario when running numbers/financial modeling unless you know 100% what the figure will be.

We do it using the BRRRLO Model in the outskirts where @Bob Okenwa mentioned.We have a meetup tonight about analyzing numbers and 2 trainings at the end of the month on how to buy these types of properties locally.https://www.biggerpockets.com/...https://www.biggerpockets.com/...https://www.biggerpockets.com/...

I would prevent downside scenarios by running sensitivity analyses on my financial model.

Define your business plan (or goal) and then figure out which models best aligns with that plan. 

Plug into a local investors Meetup or REI and talk to those who are using the same model you are using.  

I currently operate a Scottsdale STR in a non-HOA neighborhood and use a hybrid model of high-income monthly rentals in the winter, 3-night min rentals in the "shoulder" seasons and monthly in the summer when I break even and spend zero time managing the rental.Just be aware of what's happening so you can make decisions that are best for you.