Then you have come to the right place. To my knowledge the following aftermarket parts are available for the Gen III. Is the Gen III the best off road vehicle ever made, probably not, but they are excellent vehicles. They have fine build quality typicall only found in German vehicles. The Gen III handles like a sports car rather than an SUV, but it's has amazing capability off road.
http://www.youtube.com/watch?v=3gYJCNPYJTM
Several 2-3" suspension lifts so you can fit 33-34" tires
Sliders
Lockers
ARB front bumpers
Several different brush guards
Multiple Roof Racks and roof rails
Rear ladder
Mud flaps
High performance Shocks and struts
Skid plates
Snorkel
These are the basics you will need to make a great off road rig. Most other parts are universal bolt-on parts like winches, roof top tents, HiLift, drawer systems, fridges, etc.
If you are looking for something different and want to save on the initial purchase price, the Gen III is a great option. Here is a pic of my favorite Gen III done by Arctic Trucks in Iceland. They used specially designed flares to fit the 35" tires but I like the stance. With a the addition of more expedition equipment it would be perfect.