How to finance "Portfolio/Package Deals"

Hey guys! I am looking to expand this year. I have a very general, broad question. Have you ever sold/bought a portfolio from another investor? What are some good ways to finance these deals, and what are the requirements? (I know, very very broad question) Also, when looking to buy a small apartment complex, say 5-12 units. What are the general requirements? Credit Score? Down Payment (Any options lower than 20%-40%)? What are the best ways to approach a "seller-finance deal?" I am a young guy, I own 1 boarding house. My DTI is about 35%. This does not include my rental income (1300-2000 depending on occupancy rates.) My lender credit score is about 750. I am 20 years old. What are the best options for me right Now? I really want to stick with Multi-Family, and If I buy SFR's, I want to run them as SRO's. Thanks guys I really appreciate all of the advice I can get!