Don't we already have threads about all of these? Sorry, perhaps if I were a truck guy I might remember more specifically. Here is the wiki entry for aeroshells:
Pickup truck aeroshell - EcoModder
What year Forester are you considering? I had a 1999, it developed a bad head gasket (which was common for about twenty years), and it seemed like I was repairing it every couple of months. I bought the car with 93,000 miles and only put on 10,000 myself. I had an Ultragauge, drove 55 MPH wherever possible, and only averaged 27 MPG.