Creative finance for my first deal, ADVICE PLEASE!

[Ty Zaczkowski]

Been searching for my first deal for about 2 months now and I managed to find an off market 4 plex.  Owner had a rental unit available on Craigslist so I messaged and asked if was willing to sell. Turns out he's very interested. He's looking to get 300k for the property (hoping to come down to 250k). Basically he has said he is wide open to SELLER financing. My plan is to take out a mortgage of 125k to pay off his remaining mortgage of 90k and give him a hefty down payment and have him carry the other 125k on a 15-20 year note. My main question is, is it possible for me to have the property reassessed after a couple years and pull a loan on the equity to completely pay him off and just have bank become the only financier? Just want to make sure this is a legally possible plan.  

Some more details:  the property currently has all 3 other units rented out for 800, 750 l, 565 and the 4th unit I would occupy. And be living for free. Would be cash flowing roughly 400$ a month after all expenses.