I do not want to buy a crap deal and will do my due diligence.

She has an agency of 3 agents, with a combined experience of over 95 years.  

The poor guy grew up in this house and he has concluded that being a landlord is not for him, did I mention the dog crap?

Usually this criteria is combined with length of time required to recoup their original investment, typically 2-5 years. 

In particular, if there was a preferred return and let's say the investor put in $400k, got $300k back via preferred return so far and an additional $150k back as share of remaining cash flow over the years (so he actually got $450k back in aggregate), and there is a profit when the asset is sold, would you first give the investor $100k (the remaining portion of his capital not paid via preferred return) before splitting the rest of the sale profits according to what was agreed, or would you consider that the capital was already returned (as he got a combined $450k), and proceed directly to splitting the proceeds of the sale?

There is now way to combine the payments into one.

Every partnership has these two aspects in a vast variety of combinations as to what each partner will contribute.  

I'd be surprised if your vacancy and maintenance combined are only 10% for the long haul.

In my experience a partnership can be an excellent way to combine complementary strengths in order to realize a shared vision. 

Doesn't anyone throw the ex's crap out on the front yard anymore?