I would keep the MFH I purchase and manage OOS. 

To boost bookings, connect with local realtors, engage real estate investors, partner with relocation experts and insurers, offer business rates, target healthcare travelers via Furnished Finder, collaborate with property managers, amp up online advertising, improve your listing, attend networking events, and follow up with potential clients.

obviously 150 doors on conventional loans for each would murder you so I assume that is the case and the 8 doors listed are just examples. at 150 doors you need to start talking to fund managers or heavy hitters in the multifamily space.

Looking to see what the best recommendation for a Channel Manager/PMS for running this as a business.

I have many years of managing rentals.

But when we found the issues she decided to stay back.  

If I decide to scale up one day, do you have any thoughts on bigger unit count properties and the financing options available to those instead?

Bonus points if it also is great for maintenance requests and managing current tenant communications.

First step is to have 5 years+ of experience in buying, managing and selling commercial properties.

So, if you can find a deal that works in the current environment you can manage it to be even better as time goes on.