We recommend the Panasonic Toughbooks for outside adventures. While they are quite expensive new, they are readily available for reasonable amounts when they come off lease and government agencies resell them. As of 2014, the CF-30 model is around 100-300 on eBay with a dual core CPU and 2-4 GB of RAM.
The power jack cover is almost always missing; don't worry about it, although the other port covers should be intact. The power supplies are also frequently aftermarket or missing. You can replace them with common 15.6V thinkpad supplies.
These are frequently sold without drives or OS, which is fine, but be sure that it includes a drive caddy. The caddies can cost more than the machine to replace! You can easily install a SSD, almost without any tools, into the caddy and slide it into the machine. Alternatively, there are aftermarket caddies that allow you to install an SSD into the optical drive slot.
The WAN cards and GPS are hit-or-miss. You can install a full-sized SIM and have access to the GSM network, although it won't work on the Playa.