Not all Kayaks are equal!
I am new to flyfishing but not new to kayaks. If you like to have things spread around and particularly if you want to stand up then a sit on top is for you. If you plan to cover any distance, i.e., more than a couple of miles, then a sit in sea kayak is for you. You need to consider initial stability; the wider the boat the more stable. generally 24" is a stable width. I paddle a 21" boat that is very fast and efficient, but, I wouldn't consider trying to cast anything from it. There are comercial pontoon systems that will make anything stable that can cost up to $300. It is possible to jurry rig a fairly decent pontoon using spare paddle and paddle floats.
Wind and other boats are big problems. Motor boats tend to cruise along w/ bow raised and have a hard time seeing anything in the water. Paddling in groups and waving paddles over head as high as you can is about all you can do. If your upper body resembles that of a gorilla then it is possible to paddle some against a 30 not wind (flag sticking straight out). For us mere mortals 10 to 15 nots is it. Dress for the water temperature not the air temperature; hypothermia is no fun.
Paddle net will review specific boats and gives good info. Also they have used boats.
Get the best paddle you can afford, it makes the whole trip more fun. If you are going on open water seriously consider getting lessons on sea kayaking; there is a lot more to it than you would think.