Bank Loan for a Down Payment?!

[Kelby Schimming]

BP,

I have a deal in the works and the seller is willing to finance. However, due to my inexperience, he is requesting a down payment around 300k. I will also need another 200k for closing costs. It is an off market $8mil portfolio. He has agreed to interest only payments for 5 years which would allow for significant cashflow that would be banked to pay off lender asap. I would be able to pay back the loan within 3 years easy.  

Could I get a bank to loan me the 500k? Is this something banks do? 

Or am I best off finding a private lender? If I find a private lender, how could I offer the lender security in their investment? 

Thanks in advance!