The taxi fare from airport to Panwan depending on the type of vehicle, for joined van is hundred something. For limo is about 400 bath approx.
About tour reservation, you don't need to book in advance as there are heaps of speedboat operators selling cheap Phi Phi Packages. If you dont go during New Year, then you should not be worry at all. Also, you dont need to go all the way from Panwa to Patong to find good agents selling cheap boat trips because I believe that the tour counter at Panwa Beach resort will offer you quite similar price. But you want to check and compare the price, it is better off to go to any established travel agency in Phuket Town, they will offer you good bargains on trips to Phi Phi. As Panwa is only about 20 minutes away from Phuket Town so it is better than going all the way to Patong.