If you or your spouse are not an EU citizen, you will need a visa and residence card in order to spend more than 90 days in Spain (indeed in the entire Schengen Area which functions as a common border).
In case you will need a visa, New Life Spain can help you navigate the requirements for the type of visa that's right for you.
How does taxation work in Spain?
The basic premise is that you would be taxed as a Spanish resident if you spend more than 183 calendar days a year in Spain. New Life Spain can refer you to English Speaking accountants that can ensure the most tax advantaged situation in a wide variety of circumstances.
Can I transfer my driver's license?
If you are from the UK you absolutely can (note this is written before Brexit is final and that).
If you are from the US, the answer is unfortunately not. If you wish to continue driving, you will have to get a new license. Your current license will be valid for 6 months from arrival. The international driving permit is, unfortunately, just a translation of your current license and carries no legal weight on its own.
New Life Spain can help guide you through the process of getting a new driver's license should you want that option.