Tenant occupied with no lease

[Jeffrey Lecroy]

I am thinking of purchasing a home from a friend in South Carolina . The property is occupied by a gentleman who has rented the property for many years . My friends mom owned the property until she passed . My friend acquired the property thru probate. He would like to cash out. I meet the renter he seems to be a very nice gentleman and maintains the property fairly well and has always paid on time  although about 150.oo per month below market. My concern and question is . There is no lease . Can I present a lease after I close on the property and can I put in the lease that the rent will increase by X amount every year ? Any advice or comments appreciated