For example, propose a smaller initial down payment and larger monthly payments, which could free up some capital for the rehab budget.Partner-Investor:While not your preference, finding a partner-investor could be a viable option.

Try to network locally with some contractors that have done the kind of work that you're hoping to do and ask for some of their most recent proposals. 

These websites often include project proposals, public notices, and contact information for city planners.Local Newspapers and Publications: Stay updated with local newspapers, magazines, and community newsletters.

My husband and I would like to propose some creative financing options...potentially offering a lease-to-own style option?

The financials of the property and applicants investing experience / credit history will factor into the proposed terms for the purchase. 

Additionally, I proposed introducing new retail products, and the owner has offered me a 40% share in return for my expertise in retail.

In a recession the STR market will be the first to get hit and your proposal doesn't have an exit strategy as the value of the property along with daily rates would decrease substantially. 

One more thing, at my work, when we do cost plus we have to come up with an estimate and get approval to exceed the estimate which is something I would suggest adding to your proposals.

Usually a 1 to 1 or 1.25 is standard..or the $1k needs to be sufficient to cover the PITI of the proposed loan.

This is a good model but doesn't account for a return of the $500k your parents will contribute..For example if the property generates $10k per month with $6k in PITI & operating expenses the proposed return for your parents would be: $2k and $1k for each of the minority partners.