Jassiem,
Great questions man and good for you and your girlfriend for doing what most people wouldn't in order to start on your REI journey. House hacking is a GREAT strategy when purchasing correctly. Without knowing anything about the property and the specific area, I can say that small multifamily deals with good returns are hard to come by in this market. A deal with 18% CoC Returns is decent by most standards and I would say this is a buy but do keep in mind that you will not see these returns immediately while house hacking until you renovate both sides, move out and place tenants (12-16 months). I also think a more important questions would be are your numbers accurate for rent, repairs and expenses. I would consult with a trusted, local, investor-friendly agent or property manager about rents for duplexes in that area as well as how much their clients in that area are paying in expenses.
Offer your 70% number with a thorough explanation of how you came up with that number and if the seller and listing agent are investor-minded and truly motivated, they should not take offense. The worst they can do is say no and ask you to come up more. At that point if you're comfortable with and can afford the associated costs, I would then offer your 80-85% number letting them know that this is the absolute best you can do without risking losing money from Day 1.
Good luck and happy investing!