I also just received an email from the buyer this morning stating he is holding out this month's payment and future payments in case he needs them for legal defense.

What are the legalities of it?

That's very affordable.You should be able to check comparable rents on-line to find out what rents you could reasonably get for those two units.Regarding legality of two units on one parcel of land, check with the local Building Inspector, or Zoning Board.If it wasn't a three-hour drive from me, I'd buy it.

I remember reading someone's post here on BP that this other landlord has a clause IN THE LEASE that states that any money collected from the tenant goes to pay for damages first, next to pay late fees and past rents and finally to pay current rent.In your case, when the tenant paid the rent late, the first dollars ($20 in your case) would go to pay the late fees thus leaving the tenant $20 short on current month's rent IF THAT CLAUSE IS IN THE LEASE.Not legal advice.My vote: evict.Keep reading landlording posts here on BP to get more tips.  

How long before you turn 18 and can legally sign a contract?

Their lives are almost always in turmoil....they  are almost always broke, in the process of Divorce, in the middle of legal action and probably living off of the government.

Is this legal??  

I am looking to purchase the property as an investor and am willing to wait out the First Look period to avoid any legal issues. 

IMO, FICO scores are so generic that while you can filter out, you may also be throwing the baby out with the water - - missing tenants that are just recovering from messy lives.Before pulling a CR/BG report, I vet the previous landlords, employment and income, thus saving expense for a non-starter.Then, the CR (if you get a real one from Trans Union/Experion) will have all the accounts and their payment histories.  

Learn how a real estate transaction works and the legalities of contracts.