I will have about 15k-20k in liquid Cash when I turn 18. 

The two main vehicles there are DSTs (mentioned above) or using a Tenant In Common deal that may have better returns and liquidity.

People confuse liquidity with safety in regards to risk.

Maybe someone to spray. 

 @Kristen Ray Home Depot, Lowes, Lumber Liquidators.

I'm exploring different funding options and would love advice on using other people's money (OPM), leveraging business credit, or possibly borrowing against my 401k to keep more of my liquid cash.

You'll want a great Realtor, contractor, and Property Manager.2) Ensure you have enough liquidity. 

The retirement/financial independence equation he uses is a paid off primary home, 500k in liquid assets (usually retirement, cash, and after tax brokerage account), and more than 1 source of income (part time job, real estate, social security for those of that age, dividends, etc.).

Most lenders will want to see your track record, net worth, creditworthiness, and liquidity to qualify you.

So most likely you'd have to come out of pocket or find a partner that wants in on the deal to provide the liquidity.