The more efficient solution might be to purchase from Grandma with the 25% down..you ca use the income to help qualify and obtain gift funds or other sources for the down payment.

(Higher than normal rent.) and I will give you the option to buy the property in 5 years for selling price minus x, maybe minus some of Y. 

Ultimately want to focus on building a new source of passive income.

Depending on income and credit there are always credit card, personal line of credit and personal loan options available out there

I would be most worried about finding good lead sources, learning how handle the leads you get, and figuring out how to properly comp / analyze properties . 

I have found a property that I like for my new primary residence and I am trying to see if the seller is open to a longer duration close where I rent from the seller for 90-120 days to allow enough time for the investment properties to get leased.I am also considering asking if they would be willing to explore a seller-financed option but I don't know how to structure that so that a regular seller might find that more attractive than a regular closing. 

Connect with @Joseph Chiofalo for some VA options of 1-4 units.

I am just trying to see what my options are when it comes to purchasing the house and turning it into an investment down the road.

The 15yr would net right around 0 cashflow, so not an option. 

Cant speak to your other options.