Rent to buy, how to give a fair offer

My Family and I have lived in ybor for a year came June. We have had a crazy and unbelievable first year with the hurricanes last year we thought i would be a bad idea to stay but after that the kids and wife loves the weather. We were joking with the landlord about buying the property from him. He said he likes us very much and that he is an investor first. 

So now I'm thinking more seriously about buying it from him. I know some of the issues the home has. It has a history of flood and fire damage as well as piping and sewage problems. I'm not sure about how old anything else is but it has been remodeled twice before my landlord bought it.

I want to give him a fair and reasonable offer on the property. I do have some financial barriers and credit issues so hopefully seller finance or some other form of creative finance can be considered. But how do I know what to appraise and how can I get a good deal without offending him. Help please lol!!!