Multifamily home contract question

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I'm looking at a multi family home with 6 units, it's not on the market but the owner will sell. I'm hoping I can take knowledge form some of the books I've aquired and bounce things off of the bigger pockets forums to get me through this process.  I'm currently reading the book on rental property investing from the bigger pockets hosts and am getting a ton of great great info from this book. My question, is in regards to finding someone who can produce a contract. Who will have my best interest's in mind? What is the best way and by whom to produce a contract that includes all the compex factors that come in to play when attempting to purchase a 6 unit complex that is currently fully rented? Should I seek advice from a real estate agent, a real estate attorney, a commercial real estate agent, or someone else?

Thanks