And I’m talking $100k+ in damages

I know legally I can evict, take him to small claims for past rent, late fees, and damages to the property.

If it’s an upper level I’d do it now before I leaks in to a lower unit and does damage. 

A judge won't take that text as a binding agreement especially since you denied the prospects for multiple legitimate reasons. 

It can also get you kicked out and liable for any damages the airbnb people cause, not to mention I imagine it is against Airbnb TOS.

The main issue I see in the history of the house is a massive water damage in 2015 which ended up around 40k in costs.

See below for information directly from seller:"2 BR / 1 BAOn a Lease/Purchase Contract for $45,000Tenants paid a down payment of $1350, Leaving a balance of $43,650Contract is at 180 months at 10.6% on $43,650 = $485.22/moThe Buyers are paying $112/week and usually pay every two weeks at $224, deposited into my bank account.As of now, the balance of the contract is $35,300.If, the Buyers pay the remaining 117 months, the total remaining is $56,770.74The Buyers love the location and the house for it is next door to the wife's parents and want to stay and finish the purchase.There is however some damage to the house of $7800, Quoted by ******.

I did mention my brother in law bread and butter is flipping lots, but he has the money to go in and not feel and damage which is why he can't really help with the financing part of my build.

How attractive could it be for prospects? 

The unoccupied unit has some bedroom ceiling damage, and need new countertops