Hi Tony,
My standard comments would be:
If you are looking for your first fishing yak, suggest you checkout the "For Sale"/"Classifieds" sections of the following Australian kayak fishing web sites. Always some good deals to be had there: www.kfdu.com.au, www.akff.net. Otherwise checkout Trading Post online and E-Bay for well priced new or good second hand yaks.
Not sure what size/weight you are, but consider the kayak's "capacity" when shopping around for the yak. For example I weight 110kgs. So I need a yak that can handle 110kgs plus say 20 kgs for my gear (rods, tackle, water bottles, a sounder etc.). It adds up pretty quickly. If you run too close to the stated capacity of the yak you'll be sitting low in the water and will need to expend more energy to move through the water, which means you'll get tired quickly. Also if you are in rough water you'll be get more water coming over the side. So think long about the capapcity of the yak.
Also if you are a larger person like me, you don't want a short yak. 3 meters minimum I reckon. Otherwise it will be difficult to get up and move through the water.
As for dealing with mud crabs and crab pots, then you might need to look at something that has sufficient surface area to transport these.
Cheers,
Pete