The last time some of my friends tried doing an automatic control system, the plane turned straight toward the flight line and tried to kill us all!
Unless you have extensive experience designing them, I would recommend going with a kit plane for hardware rather than trying to build one from scratch out of foam boards. The reason for this is that you will start out with a design you know is flyable and has the stability properties you want. One of the classic errors in model-scale UAV design I've seen people make is trying to design the craft from scratch only to discover that their control surfaces are poorly sized, the thing is dynamically unstable, and it requires hand-made spare parts after every flight.
I think an ideal platform for a UAV like you describe would be a foam flying wing with maybe a 3-4 foot wingspan. The flying wing design would at least in theory allow you to decouple some equations which would be difficult to do in traditional fused aircraft and impossible to do in helicopters. Also, unibody construction makes it easier to land without landing gear. Landing without some pretty complex rangefinding hardware is tough, even for a computer system. Doing a skid landing on that huge wing surface with a rear-facing prop will add some margin of error to your landing sequence. If possible, get an ARF (Almost Ready to Fly) model. They come with airframe, power system, and sometimes all the servos. All you need to add is the radio equipment (I assume you are going to have a manual override backup. No, really. You're going to want a manual override.). Expanded polypropylene foam is actually more durable than a lot of people give it credit for, and replacement parts for these aircraft are easy to find.