If your net monthly positive cashflow is 2.5x your personal housing expense plus all other non-mortgage consumer monthly debt obligations, and you are buying cashflow positive real estate, you can get a normal Fannie loan.

Does the agreement to assign contracts need to be notorized in order to be a binding legal document?

I need guidance with the legalities and structure of the deal. 

The property is in a popular location so I have experienced significant appreciation since buying it and the debt pay down is great, but it will be the only property I buy with a 15 year mortgage. 

Though the debt is interest free, it still weighs on us especially since it's family.

LLC with give you no tax benefit but a legal protection.

Bought it for 56k; paid off student loan and car debt.

Also, think of alternative sources of funding: maybe instead of debt, offer equity to your potential partner(s) or ask sellers about seller financing.

Supply and demand as well as the legality.

I think just about any credit card company will allow you to make any legal purchase within your credit limit.Two things to be wary of: 1) you'll need either One Card with a HUGE limit ($100k?