Owner Financing with existing mortgage in TX or other possible options

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I have an off market deal that came my way and need some advice on possible ways to finance/structure this deal. The property is a 4 plex and I actually own the 4 plex next to it, so I know it is a good property/area and I have already been able to increase rents significantly in my property so I know I could do the same with this one. 

My issue is that I do not have the capital for a conventional down payment, so I asked the owner if they would be open to owner financing. The owner currently has a mortgage on the property, what would be the best way in Texas to structure owner financing with an existing mortgage? 

The owner is looking to sell the property for $180K, current gross income is $2400. I offered the owner $190-200K, 3.5% interest with 5 year balloon payment.  

If the owner is uncomfortable with owner financing and structuring it with an existing mortgage what are other ways that I could finance this deal to make it happen?

If I were to raise money how could I structure it, (LLC, LLP) that would be cost effective and make sense. Is that even the right way to do it? Not sure what would be best in regards to raising money.

Any other advice on ways to make this deal happen please let me know. 

Thank you