I'm looking to invest between $50k and $75k as the down payment. 

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. 

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

Some special assessments are already board-approved for 2025 and 2026 to help raise capital for such items, however I know when the time comes there will be a loan which likely demands a higher monthly payment for owners -- reducing my cashflow. - Based on sale of other units in the complex, I believe my current equity to be roughly $160,000, a little more than 50% equity. - The condo is currently leased through April 30, 2025. - August 2025 will be 3 years since I moved out.

You should find out what percentage of the rent is being paid by S8 and by the tenant.Tenants have to sign the rent increase and some won't because they don't want their payment portion increasing.It's also highly unlikely a S8 office will approve the huge increase from $797 to $1300.- BTW: the $1300 includes all utilities being paid by the landlord.

Medical Payments: Provides coverage for an injury suffered on the premises.

This would raise our mortgage by ~ $700/month, which is a lot, but still makes our total mortgage payment well below rental on a 4bd, 2 bath.So, my question for the great minds of BP is, is this a good idea?

Come up with terms (Down Payment, Interest Rate, and Amortization Schedule) such as a 5% Down Payment, a 6% Interest Rate, and 30 Year Amortization Schedule.

@Robert StephensonIf you have enough equity in your primary or rental you can get a HELOC or HELOAN and use the proceeds as a down payment to buy another rental.

Here’s the situation:Purchase Price: $540,000Down Payment: 20%Mortgage Rate: 7.125% (30-year fixed)Monthly Expenses (Mortgage, HOA, Taxes, Insurance): $3,706Realistic Max Rent: $3,000As you can see, I’m currently short $706/month even with max rent.