You're new mortgage, taxes, insurance, and hoa will equal $800, so you'll have a positive $200 cash flow each month.

Realty Transfer Tax. 

If they charge more its considered an illegal tax.

How do you effectively navigate property tax assessments and appeals to ensure accurate valuation and taxation?

Do remember when thinking to MTR to calculate in 7 days and less avg stays to qualify for material participation when self-managing STR for your tax year deductions as a business against your W2 wages.

If there is something special in your scenario that is causing the issue you always have non-QM products to fall back on like DSCR (qualify based on the asset, only works for investment properties) or Bank Statement loans (qualify based on income, no taxes required, great for self employed).

Does time taking care of (mowing/landscaping) and managing (keeping records, paying HOA dues, paying taxes) my vacant parcels count towards my 750 hours of being a real estate professional?

Sure, unless you 1031 and defer the tax liability. 

Better to start on sound legal and tax footing.

I know there are certain liability protections, but I'm more interested in the tax implications.