So now that the waters have been sufficiently muddied, the choice is yours.

Also, if the structure does not sit on a structurally sufficient foundation it is personal property - like a mobile home - rather than real property. 

@Arthur LeaoThe Roth IRA likely does not factor into your strategy as you have described it.The Roth IRA is not sufficient to purchase a property outright or with a non-recourse loan.The Roth IRA cannot be combined with personal funds to create a deal, since it is not sufficient to do a deal on it's own.It sounds as if you want access to the income.While an IRA may invest in real property, doing so is a way to diversify the IRA savings, not a way to bring that IRA money into your real estate deals.

The contingency inspection was sufficient for me as a buyer. 

Or is only an LLC sufficient?

They 5 are: Character (reliability, integrity), Capacity (sufficient income or cash flow to service the debt), Capital (the borrower's net worth or how much of their own money they put into the deal), Collateral (the value of the assets that secure the debt), and Conditions (the borrower's situation and the state of the economy or local real estate market).

The idea that a vacant house has enough equity in it for you (or in your case an investor) to purchase it, paying off the mortgage/mortgages and any an all liens and still have sufficient equity in it makes no sense.  

In couple years we are looking to purchase a house, would it be sufficient for our landlord to provide a letter to our future lender that we paid on time?

Specifically, they will be looking for capital for a down payment and operating expenses and reserves, capability to obtain financing in an amount sufficient to close the deal.  

Lenders in this space tend to create their own form docs in order to sufficiently protect themselves.Title company has a certain set of docs, this usually pertains only to the Title Policy on the property.