If you prefer to maintain liquidity and have the potential to handle multiple projects, HML might be the way to go. 

I'm just about to close on my first four-unit, which is probably typical of what is available in Maine, in this type of neighborhood, at this price point - which is old housing stock, tons of deferred/poorly done maintenance, and relatively high labor/construction costs. 

We also are considering continuing to contribute to the stock portfolios and open a line of credit when it makes sense.

However, installation costs and procedures may vary, so it's worth getting quotes from both if you're not doing the work yourself.As a hard money lender specializing in fix and flip deals, I've seen successful investors use a mix of these sources to optimize their budgets while maintaining quality.

I still underestimate how much it will cost to maintain my properties.

Years ago, I bought into the Motley Fool stock advisory service only to get worse returns than I had been doing on my own (not that mine were great.)

I purchased a well-maintained single-family home in a growing suburban community.

Maintain a fortress balance sheet; therefore your strength as a guarantor and keep most if not all of the equity in your deals. 

Can I hire a property management company to run my airbnb or can I at least hire a company for cleaning and maintaining?

They move out ASAP and they are responsible to maintain utilities and keep paying rent until you find a new tenant.