Tenants without a Lease?!?!

[Stephen Bruce]

Good morning! Here's the situation:

I'm under contract on a quadplex. Current owner/landlord is terrible at his job. He says he does not have a lease signed for 3 out of 4 units. It is "word of mouth." He claims they all pay accurately and on time. I am going to do extensive repairs to the complex meaning I will have to remove these tenants. 

Since there is no lease, does anyone foresee legal issues with removal? 

Do the tenants have any grounds to fight me on kicking them out? 

How do you recommend I go about this if you predict complications? 

Thanks in advance for your advice!