I dont understand the trend towards GPRS. If you are going to be 1 or 2 km apart at the most GPRS would work fine. CB would work better and there is lots of good used stuff to be found. The much lower frequency of CB works much better in dense forest conditions and fog.
This is probably the part of the thread that a ham chimes in and suggests real licensed radios. I think it's only a weekend course now, then you can use real radios. I would go that route but the guys I go out with have no interest.
We are going to run marine radios this fall, which is legal as we will be in canoes this trip. I am going to be making some good antennas out of copper pipe that will be mounted to the boats. We need to extend the range of our battery operated handhelds. With 5 watts with the stock little rubber duckies we are getting 2 km under bad conditions and up to 8 under ideal conditions. This is on the marine/VHF band.