how to put #'s together for seller financing options

[Nick S.]

i'm working to negotiate on a property from a friend (seller). the seller has been fairly set on their price, but they also are tired of sitting on a vacant property for the past 2 years due to lots of city violations from the property being a rental. nothing has been done to the property for those 2 years so there's a city violation notice on it. property now needs essentially a gut rehab.

property value as is realistically around 85k, but seller has been, imo unreasonably, wanting 115k for a while. however there is a mortgage on the property (so not free and clear) and i know one of the things that the seller wants to do with the money is to pay off the mortgage. i have no idea what the mortgage amount is. i also am confident there's no other issues such as tax liens or back taxes.

repair around 60k

arv around 185. my primary goal is to flip after around 3 months of work

i do not want to do a 203k loan because of the paperwork mess that comes with it and a conventional loan is absolutely not possible. given the current conventional loan rate is 3%. i would love some help on the numbers i can present. i just do not know how to calculate the monthly payment that i would ultimately pay the seller. something like this:

1) cash 50k

2) 70k(?) @ x% /30 Years with 10 year ballon if desired by him. $x month.
2) 90k(?) @ x% /30 years ...........same...............
3) 115(?) @ x% /30 years ........... same.............

thanks