What would be good terms for a partner in an investment?

[Michael Ramirez]

So I made another post asking questions about a hoarding house in San Diego.

my concern now is finding a partner for the capital side of the transaction.

These numbers are not concrete as Ive not had a home inspection or contractor walk through the property with me. So let's consider this an example. 

a home that after the rehab will be worth about 610k conservatively

The seller has a need for long term payments with a 40 year term. This is necessary to supplement her income and not cause a large tax implication. No balloon in sight because of it.

The price we have discussed has been a 300k seller financed mortgage with 3.75 interest.

PMI would be about 1200 a month.

the plan for the property would be to convert the two car detached garage into a 2/1-1 1/2 roughly 750-800 sq feet ADU. With it's own parking spots and 1/3 of the backyard privately fenced in. (Theres a fantastic hospital less than a mile away so maybe travel nurse housing is an option here for more income)

Market rents for the 2/2 house would be between 1900-2200

For the ADU its be about 1500-1800

Total rent range 3400-4000

Property taxes are 500-550 a month

Rough cash flow 1100-1250 (year one)

The inside of the property would need to have: 

-All the flooring replaced. 

-The ventilation system would need replacing. (Indoor smoker)

-Either paint or replace all the drywall (again because of the smoking)

-Replace the water heater.

-Replace the heater/fan.

-Bathroom remodels

-Would prefer to remodel the kitchen because of its layout but also isnt the end of the world to not. 

During this process I'd do all that is needed of me that doesnt require a tradesmen (flooring, painting, drywall work, installs, running materials, etc) 

After all this preliminary info I'd like to know the communities opinions on how this may be structured to benefit; the capital partner, the seller, and myself. I'm new to this process and am open to all the pointers,  criticisms and concerns you all have thank you!