Optimally liquid reserves but often to include retirement accounts or other investment account statements

It is in a town where lots of people go to retire, so the average age is 60 years old.  

Not a high cost of debt/capital at all, so we are not concerned about the balloon payment.Regarding long term goals, we plan on building slow and steady over the next 10 years, but then have them all paid off within about 20 years for retirement income from the rentals.

What will this deal look like in 20 years (or whatever your retirement timeline is)?

Over the last 6 years I have been retired as a real estate professional exclusively handling my own properties and living on the income generated.I don't regret the decision and will eventually fully retire.

are you already in a strong financial position - emergency fund, savings, maybe a retirement account?

My real estate journey started two years ago when my parents passed and left me what was to be their unrealized retirement property.

In fact, when I retired from the W2 I told people I was downsizing to one full-time job (my RE). 

For more details refer to the link above.No, you can not invest your retirement funds into the LLC that you personally own (with or without a partner), this would be considered prohibited transaction per IRS:https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-prohibited-transactions Regardless if you use an IRA or 401k - all investments you make with your retirement funds must be passive in order to avoid taxation. 

Retirement Accounts: Certain retirement accounts, like a 401(k) in the USA or an RRSP in Canada, allow for borrowing or withdrawing funds for home purchases.