Tenant is out of the home in a hotel, due to the extreme Florida heat. 

That's step #1.Far too many people want to run a marathon so the first thing they do is buy some expensive running shoes, moisture-wicking shirts, a $300 fitness tracking watch, a $30 app, a 60-pack of energy gel, etc.

Her old lease reads "The tenant will pay all utilities, except for the following which will be paid by the landlord: Oil heat, water & sewer."

I see no reason not to toss her but wouldn't spend a lot of your energy on thinking about her right now.

All tenants are reportedly current on rents The current owner appears to have bought it at $179k in fall 2018 and reportedly put $100k into it ( a new roof, new insulation, new dual pane, energy efficient windows, fresh interior and exterior paint, shaker cabinets and counters, new tile and carpet flooring throughout, and electrical/plumbing improvements. ) At the asking price, with 20% down, assuming $3k in repairs it would give about $1100 in cashflow (above all expenses) which would be 16.5 % cash on cash return. 

The home has a wood fireplace in the basement, and electric radiant heat upstairs.

That includes, heat, hot water and electricity.

Yes you can charge by subcooling and super heat but the only way to charge it to factory design is by weight.

I now have very little income but want to focus all my energy on building a rental portfolio to eventually replace her income.

Using Time/Energy Effectively