Buying a property with tenants on verbal lease

I currently have a Quadraplex under contract that has existing tenants in all four apartments. The neighborhood is somewhere between a C and a D so I am not confident I am acquiring the best tenants. The only landlord did collect security deposits but only has a verbal lease with these tenants. What I'm wondering is what would you have the tenants sign before buying the property? I found a good estoppel on biggerpockets but it doesn't list important things that would be found in a lease. 

Also, right now one of the tenants is paying significantly below market rents but is taking care of the lawn care and snow removal. I would like to keep this agreement with him, but am curious if anyone has any input on this? Any information you can give me is appreciated, I am not closing for 3 weeks so I still have a little time to get these things taken care of.