What you cannot do is use the funds that the C Buyer brings to escrow and pass those funds through escrow and use those funds to pay off the A Seller. 

If to big, slice off the good farm ground and sell to the neighbor farmer.  8.  

By the time we are our parents' age, our properties could be paid off/cash flow well, and we are sitting in an amazing position. 

I would get a quote from 3 contractors off of google and go from there

It sounds like your off to the right start -- but watching some videos and doing some research -- keep doing that.

You are off to start interviewing PM's to get a better understanding of what market rents are and if these folks are qualified to rent your property.

If I use seller financing to own a property, do I build equity even if I sell before the mortgage is paid off?

I need to find resources for off market deals or below market deals.

But, I have come across an off-market 55+ community which I do not have any experience with.

I have a million classified posts showing the style we build. you'll never get the numbers at that rate. your numbers are way off and way too low. work with an experienced general contractor who knows this stuff.