I typed out a long reply, then lost it...
The quick answer is that the assessed values are probably all over the place due to the Florida homestead exemption which limits increasing the appraised value by no more than 3% per year in cases which qualify. So, some homes' appraised values are probably way below market value, since the appraisals were not allowed to keep up,with the real rate of property appreciation.
The homestead exemption may also allow you to have your property's appraised value reduced by $50k if you live there, rather than rent it out.
I would discuss this with the
assessor's office in Orange County. They could probably give you the specifics on how, if, and when a purchase would trigger a reappraisal or loss of the homestead exemption which could result in a significant increase in taxes the following year.
Millage rate is just a multiplier. Assessed value x mill rate = your property tax liability.
Mill rate is usually determined by dividing the budgeted revenue for the year by the total assessed value of all properties in the jurisdiction. Assessed value determines your property's share in covering the budget, and the homestead exemption basically means that in a rapidly appreciating market, long-term residents pay a smaller share of taxes than newcomers and investors, by artificially lowering their assessed value.
