But I want to understand:How quickly do rental units typically get leased?

A HELOC could help temporarily if you plan to payoff the balance quickly and are planning to save on monthly payments in the short term.  

Engaging with seasoned investors and participating in discussions will expand your knowledge quickly.

Hindsight Critique•Continue to build relationships to increase deal flow•Property manager (Mercer Hughes) is critical to completing renovations and quick tenant placement•Continue to build relationships with private lenders and personal investors•Have cash available to make quick competitive offersDid you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

A quick check of new listings results in many listing that seem well below market rates.   

Hindsight Critique•Continue to build relationships to increase deal flow•Property manager (Mercer Hughes) is critical to completing renovations and quick tenant placement•Continue to build relationships with private lenders and personal investors•Have cash available to make quick competitive offers Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

I would encourage the use of a warranty deed to the situation that @Steve Vaughan mentioned above.

What pitfalls or challenges should I be aware of when scaling up quickly?

You want to be able to get rid of a tenant who is causing problems in the home asap as it can quickly create a hostile environment for the other 4 people living there where now you have all 5 people contemplating wanting to move. 

After reviewing the cost to rehab and the area of town, I decided to Wholesale for a quick profit.Sold it on a Real Estate Contract (Owner Financing) with $100k down, the balance is financed at 10% for 13 months, then the loan is called.