1st Deal Ever Seller Financing Need Help

Hello I am new to BiggerPockets found them while just trying to figure out how to qualify for a personal residence and it has now turned my life completely around! 

I found a 4-Plex for sale by owner and I asked if they would consider owner financing they came back and said they would consider a Land contract, 30 year with 5 year balloon, 6% interest, $8,500 down. Sell price $85,000 firm for Land Contract. I need about 6 more months to be able to qualify for conventional financing but I do plan on taking a mortgage to pay the owner financing  debt.

I have run the numbers with the information given for the scenario of the rent revenue being Gross or After expenses just in case but waiting on them to confirm if the rent revenue is gross or after expenses: 

Rent Revenue - $23,040/yr

Expenses (approx. annual) 

Gas - $3,600
Water - $2,000
Electric paid by tenants
Waste Mgmt - $780
Lawn Maint./Snow Removal - $750
Taxes - $2400
Insurance - $800

I want to know how to put this deal together?