About $900 a month after taxes/insurance/and misc repairs. 

You should make sure that the rental revenue meets all of your costs, including the mortgage payment, property taxes, insurance, maintenance, and vacancy allowance, since monthly rental prices might range from $1,000 to $1,600.Property Appreciation: Take into account the market's expected and past rates of property appreciation.

For more precise estimates of rental revenue, think about speaking with nearby property managers or real estate brokers that focus on rental properties.Closing costs: Compute the closing expenses for the duplex purchase, taking into account attorney fees, title insurance, property taxes, appraisal fees, and loan origination fees.

He is ONLY willing to sell by owner financing.I am not a tax expert in estate taxes, but my guess from knowing others who have had similar situations is that IF he sold it outright for say 300K, he might have 75K of taxes.

I thought maybe she’d like the increased income and reduced liability as she approaches 90 years old but I see the tax issues might be too big an obstacle as she does seem pretty well off. 

So far, when I look at a property I tend to look at: what did other properties in the area sell for, how are the schools (I.e if I want to attract families), how much does rent typically go for in that area, how much is property taxes in that area,etc.

I am wondering what are the tax implications for this individuals (most of them are from Spain). 

Since Florida has no income taxes...

is it illegal to sell my Tax Deed by quit quitclaim Deed?

Have you done your tax planning with a good accountant?