So here's an outline of the numbers and if someone can let me know their thoughts, I'd really appreciate the help. 

You come to my office I will outline what it takes to be a partner.

I've hard of spouses utilizing a 'separate' buying strategy but not for the reason you outline there. 

That is the average duration of a housing cycle from peak to peak as outlined by this article.  

Our rental agreement makes it very clear that only the named people can live there, gives a limit to how long guests can stay without prior landlord approval, and outlines the process for adding a new tenant or authorized occupant.

I’ve prepared a rather detailed operating agreement that outlines expectations for everyone.

My partner and I have come to an agreement with the friend on a purchase price (that they would receive when the renovated house is sold), and we will have an agreement drawn up by an attorney outlining the terms.

Is there a website, book, resource, or person that you could recommend that could outline this process start to finish? 

Maybe if we have a separate contract outlining the $80k to be paid later, we can avoid this?

You need a promissory note (which outlines all the details of the loan) and either a mortgage or deed of trust, depending on which is used in your state.