Potential Purchase Questions/Advice needed *Newbie*

[Elgin Swift]

Greetings all... .This is my first post here although I have been lurking for a few months. I currently have one propery under my belt (besides my residence of course) but I am extremely wet behind the ears and could use some input advice regarding a property I am considering. 

I looked at a property today, allow me to lay out the nuts and bolts of the property..

It's a 2 family ( 1st unit on the main floor, 2nd unit takes up 2nd and 3rd floor)

Upstate NY (about 1 hour from NYC)

Asking price $99,000

Property tax $4,300

Rental Income $1,680 per month

Tenants all pay their own utilities

I was planning on putting down the standard 20% down and.  Financing the rest when I run quick calculations the numbers seem to work pretty well in my favor. 

 Now the property is in a rough part of town which probably would turn a lot of buyers away but I grew up in the inner city so it does not bother me as much especially considering the upside if they ever revitalize the neighborhood. 

I did go see the property today and this is where it gets interesting.  I met with the property management representative because my agent was busy with a closing. She showed me both units. First unit was cut and dry, stand unit. Very well maintained. 2nd unit was also very well maintained but the person who rents it rents some of the rooms out himself to cover the rent. I'm not sure if this is even legal or not or creates any issues that I need too be awar of (which is why I'm here lol ) it did catch me off guard but she said that the unit has been rented by the same individual for 6 years without any issues regarding payment of rent, etc. I asked about leather I current leases she said both are "oral lease agreements" meaning month to month without a contract/lease agreement. Not sure what type of problems that can cause or not for myself either.

I'm curious to how do I verify that the tenants are actually good tenants in term of payment of rent. Is there any way to make sure I'm not walking into a situation where the tenants are bad tenants but being advertised as good ones? What are your thoughts on the rental arrangements as they stand? Over all opinion?

Any input is appreciated. 

-Elgin