You need to have a formal Buy-Sell Agreement.

It's important that you realize that if he passes away without a formal buyout agreement in place, then you are going to be in business with all of his children. 

You don't communicate if you don't have to, and you do nothing but give formalities.

Building off of the previous reply from Kevin,yes it's the home was built prior to 1938 generally a CO is not required and you can get a Letter of No Objection (LNO) from the Department of buildings though it may take some time to have that produced (Can take months)Important to note that here in NYC Department of Buildings (DOB) has final say about the classification of a building and it's Occupancy.Do you need either a CO or LNO to be generated if one does not exist and there's no formal DOB classification?

@Alex Verdugo if you are starting a new policy, meaning you do not have an existing policy you can add additional buildings on to, there is no way to get an accurate premium unless you get a formal quote from a broker.There are some many variables in the underwriting process that can change insurance pricing.Lastly, the most competitive insurance companies for rentals in AZ do not have online rating systems, so you would be missing out on the best options if you don't work with a broker.Best of luck!

I never had a formal or even informal education in finance/investing. 

You would need money from a hard money or private money lender which that will be tough because they will want a formal appraisal and with auction properties you do not get to see the inside. 

If you have not  completed the formal loan pre approval process - take advantage of this time you now have to  get a  lender choice made and  then  get all your  supporting data to this lender so that they can  get the pre approval  ( not pre qualification )  solidly in place ...this  process is free ....you still have  the opportunity to  cost compare this  lender with others at this point  ...having the  solid pre approval in place may help  with  getting your offer  accepted  over another  person offer - good luck

The venmo app provides an easy method of collecting rent while providing proof to both parties without the need a formal receipt such as you would want to provide with cash payments. 

Your best option is to send them a formal letter requesting full payment of rent owed and release them from their lease.