Rentals also build equity through tenant payments and appreciation, with profits taxed at lower long-term capital gains rates when sold.

Your $80K HELOC can serve as the down payment, as most lenders require 20%-25% of total project costs upfront.

I have a 700+ credit and willing to make 25% down payment.

When revenue is low in the first couple of year, so is the interest payment.  

You all must agree to the same thing or else you will just end up fighting about what to do.Since your mom doesn't have it in a living trust, I suggest you do that right away.

I like to believe I earned their trust for when I need to borrow from them again.

In my experience that's not actually the case at least in legitimate evictions for non-payment of rent or breach of another part of the lease.  

You can allocate those proceeds any way you want - as equal down payments or buy one for cash and others with loans - It doesn't matter a bit as long as you use all of the cash and purchase at least as much as you sell. 

TRUST ME!

I offered them 10% down, interest rates favorable to me, amortized for 30 years, but with a balloon payment after 3-5 years.