I already have software that does my accounting, leases etc.

You also need to take into account how your approach will impact the tenant and what response you are likely to receive.It would be helpful to know what city and state the rental is in, as landlord-tenant law varies per jurisdiction, so what works in our area may not work in your area.So, at the start of tenancy the tenant's parents paid the first month rent and security deposit.

I am now an accountant for a state agency, and I am just tired of working for others and having nothing to show for it.

On the actual lease, I would increase the rent about $750/month for the first year to account for the $9k payment and the second year it would go back to the same rate as right now. 

Five days into her stay we get automated notifications from VRBO that the payment had been reversed and they took the money out of our account.

I'm assuming if it's a buy and hold you're accounting for the highs and lows the RE industry goes through?

We just pay for legal and accounting services, if neeeded..

Essentially your cost of money for the down payment is the after tax cost of debt...your home equity line interest rate after taxes versus the interest you are earning on your savings...I am guessing the cost of the equity line interest is more than what you are earning in the savings account...either way, I also suggest hiring a professional property manager to start with.  

Planning on converting to real estate at least a portion of the account. 

Does your complex have any extra space to possibly expand into something that will earn you more money.Have a reserve account with a balance that will help you make repairs and turnover expenses.