First time investor: Help me analyze a deal. Thank you!

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The agent calls it a "cash-cow block duplex." 

Property details below but here are my questions:

1. The current ROI is listed as 8%. For any listing, would the seller/agent be able to provide accurate paperwork to prove validity of any listed ROI, cap rate, etc?

2. Per the listing, tenants are on month to month agreement so rent could be increased to market potential quickly. If I buy and increase their rent, current tenants could leave. May invite additional costs to obtain new tenants. What is the likely scenario in listings such as this. 

3. I tried to apply the 10% rule (dividing annual rental income by purchase price). 

Current rent of $11,100 makes it less than 10% = not a solid investment 

Potential rent lists $18,000 makes greater than 10% = a solid investment. 

Are there other exceptions to this rule to consider? Is this a good rule to go by? 

Property details here: