A twintip in the 135-140cm range would be your go-to board. Generally the longer/wider the board is, the better it is toward the bottom end of your kite's range, whereas a smaller board will be better at the upper end of the kite's range. For 170 lbs I'd probably try to find "freeride" twintip around 136cm by 40cm and is fairly flat and medium flex. South Florida is super choppy so you'll either want a lot of rocker or some decent flex so your knees don't explode, but unfortunately florida is also not that windy, so high-rocker boards are kind of out of the question because they're really slow. Every brand has a general do-it-all board that goes upwind well and will serve you in your progression for quite a while.
Some people will say to get a huge board so you can learn to go upwind easier. You can do this if you want to spend the extra money on a second board. A big lightwind board will be useful if you want to try to ride here over the summer when the wind is light... a big board makes a big difference. But, they aren't much fun to ride unless you enjoy riding back and forth in straight lines, so it's up to you. A lightwind specific board would be something like the Liquid Force Overdrive, Best Breeze, and so on. Most people don't go this route, they just jump straight onto a "regular" twintip, but it may be worth it depending on your athletic ability and how quickly you feel you're learning in general.
As far as where to buy a board from, you can get away with a lot more with a board than kites, so feel free to buy something used even if it is cheap. Modern boards are generally tuned up a little better in terms of shape and channeling to help you get grip and go upwind, but it's not really a mind blowing difference. Something like this would be a good place to start looking if you have no idea: http://www.windance.com/137cm-Slingshot-Crisis/39954/
decent price and that's the board shape I am suggesting (low rocker, medium-ish flex).