I am looking towards a PPTP bridge over serial

. You only have one ethernet port, but maybe 3-6 USB ports! Bluetooth would be a great technology to use, however, I haven't found a way to create a PAN network from a computer so that tablets can connect. However, a touch screen might be a powerful weapon against the ghost of scouting!
A RasPi is a perfect choice. If you write your server in a fast language such as C++, there's no need for anything more powerful!