How to leverage this purchase

[Benjamin Morris]

An opportunity has presented itself, but I can't find a way to make the numbers work. A commercial property owner is looking to sell an asset to me for: 1.3M. I would need to pay off the current mortgage of $300K and they would owner finance the balance. They need to be out of the picture in 15 years. I laid it out in Excel that it would be interest only for the first 5 years, while I take out a mortgage for $300k; this doesn't get my monthly payments low enough, so looking for alternative suggestions to make the monthly payment lower. 

Rough income/expense numbers are: $184k income; $100k expenses [Pre-Mortgage]. 

The property has an upside of being worth ~$2M when fully rented; it currently has 7 vacancies. The owner is not actively looking for tenants. Any suggestions or ideas to make it more palatable would be appreciated. I can sell a couple of properties [within maybe a year] and net approximately $200k if that changes the outlook.