Conduct a soil study and rock to soil ratio before making plans to build.

Those types of buyers usually need longer timeframes to do soils, environmental, deal with planning/building dept/public works, etc, not to meeting survey, engineer, architect, etc.

I have found that in Texas, clay soil will cause foundation issues.

Be careful with flood insurance, it's still insurance and they'll find any way they can to weasel out of covering things (LOL, sorry insurance folks).Coming from a commercial/industrial floodplain permitting perspective, I would recommend most projects be raised to freeboard level (>6.7' in your case) at the lowest floor and engineered to pass floodwater underneath then apply for a LOMR (letter of map revision) with FEMA to remove the project from the floodplain.

Estimated $18k for total replacement.2 toilets are leaking and the floor underneath the toilet is "squishy"This is a class B rental. 4 of 6 tenants have lived there 5+ years. 

Checking with the departments one by one for each land listing that looks promising seems very inefficient.Regarding costs, i hear build costs of containers vs stick built being similar, but I had wondered if that is taking all aspects into consideration like build time, foundation and site prep requirements, etc. yes, steel cutters and welders are a special trade but the structure can arrive pre-built and potentially just sit on concrete pier foundations if the soil and zoning allows.

If TENANT(S) does not clean adequately and regularly, TENANT(S) will be liable for reasonable cleaning charges – including charges for cleaning carpets, draperies, furniture, walls, etc. that are soiled beyond normal wear (that is, wear or soiling that occurs without negligence, carelessness, accident, or abuse).

Sometimes in driving rains, the rain can get worked up underneath the shingles that doesn't happen in a standard rain.  

Five years should be plenty of time for your friend to get his feet underneath him and either pay you back, refinance all mortgages, or get another loan to retire yours. 

Almost universally no one appreciates that each states Safe Act is underneath DF and controls per state limits on if you can even do seller financing and if so what limit.