15 unit multi-family opportunity, need guidance

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This would be my first deal. Its in St Mary's County, MD, 2 hours from where I live. I've had one conversation with the owner who is very aware that I am inexperienced and new to investing, from the questions I've asked. He also knows I need to raise the money for the down payment. I called him back today wanting more details about the expenses and he was quick with his answers: 12% CAP rate, 1.2M asking and 350k down payment. Owner is retiring and willing to seller finance. What I know about the property is:

income: 14,000 to 22,000 a month,  expenses: 9,000 taxes, 8,700 trash p/u, he pays water, electric, water is not from city water.  Its 3 bldg., large 2B, 2BA & 3B, 2BA, low turnover, month to month leases: rent is based on per person, 1 person: 875.00 rent 2B, 2Ba, Den, Balcony, 135.00 more for 2nd person, etc.. same lease for everyone,

How can I show him that I am a serious buyer.  I am willing to go 80/20 with a partner just for the learning experience.  I need investor/s for the down payment. Any advice would be appreciated.

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So you wanna buy a property that fluctuates in monthly income from $14,000 to $22,000?  Why?

Why are you willing to buy at a 12% cap rate?  Do you have any cap rate comps?  

Why do you want 1t rentals 2 hours away?