NEED HELP** I'm a noob, working on 1st deal (personal)

[Alfred Vitale]

NEED HELP** I'm a noob and I am interested in selling my property in NJ outside of a traditional mortgage. I would like to sell my property for $605,000 with a down payment of 15% using sub to for an existing mortgage of $290,000 @ 3.99% with a balloon payment existing of $12,000 and then seller finance the equity at 3% for 25 years. 

After doing some more research and networking some people suggested a wraparound mortgage would be best. 

What are the pros and cons of each, I am not sure what provisions would I need to make sure I am protected and is there any advice you would give about this deal?