In a loose sense, I would call any lender a "partner" too, since they will require not only your monthly payments, and will require regular reporting as well on the operations of the property (but this is a more semantic and philosophical discussion).But, ways you can try to come up with that money: refi your home and/or investment properties.  

They sold their home and retired to a very walkable 2bed 1 bath apartment in a neighborhood they liked.

The second option would be that I flip a home.

My question is what are the types of homes that attract long term tenants in this market?

Building a home for sale is the same as house flipping.

My degree may not be my passion, but I know it will provide an income that I can leverage into something else (I've already started working).Currently, I am living in a home owned by a relative, and my rent is impressively discounted.

If it is to have a vacation home that someone else helps pay for then maybe.  

Hello, my real estate investment partner and I are in talks with a party about getting a $1-2M promissory note in place, which we would draw on to make acquisitions of 12-25 single family homes over the next year or so.

While you don’t have to continue to find home for your $ every year it is tied up so it is illiquid.

We are going to be buying property and building a spec home shortly and need to understand the pros and cons of doing this deal through an LLC vs. as individuals, from a tax perspective.