seller finance and portfolio lending

Hi, I have a retiring landlord that is selling me his portoflio. He is willing to carry 20% of the sale as a second mortgage or any other security.

To buy the portfolio I think the best lending options i have are B2R, First Key Financing and other similar portofolio lenders. However, they don't accept 2nd mortgagaes. 

I don't need the 20% for the purchase, I can get that money myself, but i don't want to miss this opportunity. Can anyone suggest a way to structure this deal?

Thanks!