Purchasing Portfolios/Portfolio Lenders

Hello! An investor in the area is looking to offload his 5 property/9 unit porfolio used for student housing. I already have a a house that is rented to students in the area and the goal was always to scale, so I am very excited about this opportunity! The investor reached out to me directly and the portfolio is not currently on the market.  The portfolio consists of 4 duplexes and 1 single family home. 

I originally thought to propose seller financing on at least some of the properties (which I know the investor/seller would have been open to) but they all 5 properties currently have a mortgage so that likely wouldn't have been an option, correct? Specifically, I am thinking about the due-on-sale clause in the loan agreements. Is there a way around this and is it worth looking into deeper. I don't want to delay too much and lose out on this opportunity. 

Because of the mortgages, it appears to me that I will have to buy this all cash. Someone I met at a local BP event suggested a contact a portfolio lender who will look at the value of the portfolio as a whole. He said the lender will probably want to see some skin in the game (I believe he mentioned 20%. Does that sound right?). My plan would be to do a cash out refi on all properties to pay the lender back or use the rental income to make my monthly payments. The portfolio is valued roughly at $2.4, maybe even higher since these properties are often treated as commerical properties given the high rental income. I'd like to purchase for $1.8-2.0 (I know that the seller turned down an offer of $1.6 about a year and a half ago). The investor/seller is very anxious to get out so he may take below market value for a quick cash sale.  

Please let me know your thoughts and if you have any insight on how porfolio lending works and what these lenders typically look for. I heard that it could be difficult to get a loan from portfolio lenders. 

Thanks in advance!