I grew up with horses in Florida. I doubt much has changed. Pros: year-round riding. No such thing as "too cold to ride." Even in summer, when it's hot, you ride in the late afternoon. Most of Florida's suburbs have wide verges alongside roads on which you can ride. I used to ride all over Hialeah and Miami Lakes with ease, simply because there was ten to twenty feet between me and the road. Many boarding barns have runouts attached to their stalls...I love this set up, although it doesn't seem as common as it used to be.
Flies are an issue, just as everywhere, but I don't recall much problem with sandfleas. It may be regional. All pastures and paddocks are sandbased, and if you feed off the ground, then yeah, sandclear is a good idea. However, I never used it and never had a problem. And every couple of years, I'd retop my paddock in fresh, white, playground sand. Great drainage and easy to pick. ;)
If you have a choice, I'd move in the spring, when temps in the north are cool but warming, after the bulk of winter coat has shed. It'll give your horse time to grow accustomed to the increased heat and humidity gradually, as the worst Florida weather hits in August and September.
Put away your blankets, cause you won't ever need them again. Ever. At most, you might use a light cover one or two days a year.
Hay is more expensive, and it's different. You don't want to buy "grass hay" in Florida, as it will usually be a variety called "coastal," and it just isn't good. I never liked it, anyway. You'll want T&A or straight timothy. T&A is what I always bought--a mix of timothy and alfalfa.
I'm a bit jealous. I never realized how fortunate I was to grow up in Florida on horseback until I moved to PA and tried to ride through the winters. I get why people turn their horses out in January and don't ride again till April. I can barely force myself onto my horse in Feb.