My experience as a one time squadron legal officer and frequent renter is that when a military member moves, they often refer another to your place.

You may have something written in the loan documents about the minimum ratio (ie. debt service coverage ratio, times interest earned, etc) that you need to meet in order to comply with their terms, so make sure you have the line items in your financial reports needed to calculate them.

As someone who has experience in this realm (however, do not take this as legal advise by any means), the cleanest way to do this is form an LLC. 

I have a friend looking for an attorney to help with the legal aspects of raising investment funds for the acquisition of a building for a gym in the DFW area.

You don't want to spend the little money you have or worse (go into debt) to pay for a guru class that doesn't work. 

In my case I’m legalizing several existing inlaws.

For 10+ years I've made most of my income from internet marketing (I've never had a "real" job), and I've done lead generation in some very competitive niches (home security, debt consolidation, etc.). 

The lease agreement would not reference the estate planning docs at all to avoid any reclassification as a purchase.Option 2 - purchase, he holds mortgage.Is it possible/legal to have a mortgage where 100% or nearly 100% of payments go to principal?

For the contract I would speak to someone that can help you with the legal terms and conditions 

If they do not have any debts and if they have a steady job, they should be more able to pay the rent than someone with a lot of debts and a credit score.