My thought is a large concrete wall higher than it and the flume as well as additional fence, soil, tree boundary.

I guess I could assume that since someone is pitching this to you that your property/soil is right for it...but that's a pretty big assumption. 

App_mode=Displ...197.172    Interest rate; calculation and minimum.(1) Real property taxes shall bear interest at the rate of 18 percent per year from the date of delinquency until a certificate is sold [...]Translation: Property owners who are delinquent on their taxes, for which a tax certificate (i.e., tax lien) is sold at auction, are charged 18% per annum.197.432    Sale of tax certificates for unpaid taxes.(6) Each certificate shall be awarded to the person who will pay the taxes, interest, costs, and charges and will demand the lowest rate of interest, not in excess of the maximum rate of interest allowed by this chapter. [...]Translation: Florida is a bid-down interest rate state. 

I form LLC's base on the amount risk/exposure I'm willing to bear.

, permit fees, engineering fees, road costs, utilities to the front entrance, soil tests etc.         

It's a little bit like the joke about how fast do you have to run from a bear?

Bear in mind what that means though - you are risking loosing as well.

Look into sugarloaf, big bear, Joshua tree, lake Isabella.

Are the loan mods that FHA does still a non-interest bearing 2nd?

In the case of a roof, etc. you could renegotiate a higher sales price (amend the contract) to cover the cost of the upgrade (if the market will bear the increase).