Keeping your cash flow separate is smart, but you might as well make that money work for you.I’d look into a high-yield savings account or money market account—at least you’ll earn some interest while keeping it accessible.

If they need access to the property immediately, you can offer to leave to them until you are ready to close. 

Regarding a HELOC, most banks will only provide a HELOC up to 75-80% of the property value, so you may not be able to access a HELOC for a while.

I'm just beginning but am considering a bigger commitment in the next five years or so by accessing some or even all retirement funds to buy a building (I'm sure people have thoughts on whether or not that is a good idea!).

This will cost you $3-$4K, depending on your state, and will allow you access to the MLS.

I’m a broker that has access to conventional, non-QM, and hard money loans.  

But, I also think the access to information today: all the books, these forums, conferences, gurus on social media...  

Finally, as an aspiring/budding lender, I don't have as much access to mentors, but then again it's easier than rehabbing/flipping and probably easier than buying and holding and managing multifamily. 

My business has access to AppFolio, but it doesn't seem to communicate with the STR side of things. 

This allows you to access funds while keeping your existing mortgages intact.Second Mortgage: Explore lenders who offer second-position loans on investment properties, though rates will be higher.Cash-Out HELOC : While traditional banks often restrict HELOCs on non-owner-occupied properties, some portfolio or private lenders may offer HELOCs for investors.With $15-20K in liquid funds, look for deals where you can negotiate terms:Seller Financing: Negotiate lower down payments or interest-only periods.Subject-To Financing: Assume the seller’s existing mortgage while covering the down payment.Lease-to-Own: Lock in the purchase price while using rental income to build equity.