This is one of those questions where paying a few bucks for professional advice might save you thousands later. 

Here are some specifics:Purchase price: $955KSeller Finance offer: 20% ($190K)Reno cost: $90K Ideally we would like to do a Morby Method where we purchase the home via conventional mortgage or hard money lender and on the back end have the seller lend us the $190K with a second position lien on the property and a 5-year payback.

As someone who invests heavily in Philadelphia, I can also say the price point you are targeting is going to lead you towards neighborhoods where the BBRRR strategy doesn't often lead to realized gains that correlate to the appraised values.

Where are you finding your deals?

I've seen deals where investors accept a deal that's not so hot just to comply with the time frames. 

That's where I am and always looking for cool ideas for the house.

I think where it will hurt you the most is if you build a garage or ADU without any permits and plan to refinance out of the FHA loan.

Most lenders want to know where the down payment funds are coming from and most lenders will shy away from originating your loans knowing the down payment money is a loan.

Dealing with a wholesaler can get you in trouble if you are confident in what and where you are looking. 

I have done many brrrr and mostly agree the sweat equity is where the quick return typically is.