As for the existing tenant, ask the seller to provide you with the lease agreement, so you know its terms. 

My non investor friends are like wow that house sold quick!..

Am I looking for a unicorn market that doesn't exist or are there investors here on BP regularly finding these deals?

My partner and I are looking to start investing in non-owner occupied SFHs and have decided to go 50/50.

If they just paid the normal amount, it would not be enough to cover the rent and I have grounds to evict for non-payment of rent. 

@Mike DornemanI agree with @Brian Garrett, and if the existing tenants don't like the new leases, they don't have to sign it or stay! 

Additionally, ask your hazard insurance carrrier how the transfer would affect your existing hazard insurance and business liability coverage.

You would also eliminate the design constraints of working with the existing structure.

I have called the state\epa and discussed what is needed for operation of treament plant, confirmed the certification and validated all but rent roll.Barring rents panning out and with the possibility of rehabbing and renting the existing vacant 4 units this appears like it will cash flow well and  have some significant upside with 10 more lots to be filled potentially. 

FIRPTA is for non-resident aliens.