Pack & Paddle in Lafayette has numerous demo days, where you can try dozens of different kayaks to see what you like. They are usually on Saturdays at Lake Martin in Breaux Bridge. Check
www.packpaddle.com the next one looks to be April 16.
You can spend anywhere from $500 to $2500 depending on what bells and whistles you want. You get what you pay for, as with everything. You can get a geat paddleboat for $1000. Peddlecraft run a bit higher. The Backpacker in Baton Rouge and masseys in New orleans also have demo days, if you are over there.
In general, I'd go with 14' or more for a fishing kayak. The longer and narrower the kayak, the faster it will be for long paddles on Big lake. A Tarpon 14' or Manta Ray 14' are great fishing boats. 12' kayaks are OK for river trips, and smaller people, but they don't track as well nor hold as much gear. If you feel the need to stand, a wider boat is better. The Native Ultimate and WS Commander both are wide enough to stand and fish, and P&P has them. I think P&P also carries the Diablo, which you can stand in. However, don't expect a wide boat to be fast. I highly recommend that you try before you buy.