Once you have the required R/C gear you now need to decide on what type of aircraft you would like to use.  You can essentially use any RC aircraft you want and turn it into a UAV.  However this page will give you some tips to help you choose the best possible aircraft platform.

Petrol or Electric?
Petrol aircraft are generally used for larger models and have the advantage of being able to fly longer than electric aircraft.  Since electric aircraft are much simpler to build and fly we will be focusing on electric aircraft.

These aircraft are more suited for beginners who have never flown an RC aircraft before as they are more stable that flying wings.  RC aircraft with a pusher motor as shown above are great because the propeller does not get in the way of your on board camera (if you are using one).  Aslo if you crash the propeller or motor shaft does not get bent as easily if the motor was attached to the front in a more traditional configuration.

Flying wing aircraft are popular for UAV's because they are easy to build, and only require two control surfaces which means less parts to go wrong.  Another nice thing about flying wings is that they are easier to transport and store compared to other aircraft.  Typically these are harder to fly compared to conventional type aircraft

The most important feature of your RC aircraft is the cockpit space.  You will need to find room in your aircraft for your autopilot.  Some autopilots are smaller than others but its still important that your aircraft has enough space to accommodate this.

You can always try to modify your aircraft by cutting away the foam to get the autopilot to fit, but this is not always a good idea as it can weaken your aircraft

This might sound silly, but it is very important that you can fly your aircraft without any autopilot.  The reasons for this is because things do go wrong (set a wrong waypoint and your aircraft is heading straight for a tree) and you will need to switch to manual mode to avoid the crash.  This is important as eventually your RC aircraft will be carrying lots of expensive gear and you dont want to loose it all because you cant fly your aircraft.  Also most pilots usualy land thier aircraft manually despite most systems being capable of autolanding (generaly human pilots land better than the autopilot so it means your aicraft lasts longer)