Ricky327,
Yes i agree ripping the lon / lat data form the text message would be the best option and most compatible such as live update via gprs to a web page will equally be as good.
as for credit you can request cordinates via the dtmf functions call up and manually request an update you could specify the update time or just a one off position.
GPS last position i agree again this will allow for last position to be recieved and will disallow the need for another rf interface, as for moving the vehicle in a truck ofcourse this will be a problem..... lets hope there arnt many clever thiefs out there.
If i was to use a text msg to turn on and off devices from my phone to the tracking phone i would need to make sure that syntax was correct for every text i sent plus making sure that text has arrived safely sometimes a big delay can take place, also i would need to purchase a RS232 gsm modem that allows for the sms data to be extracted via AT cmds, having it dtmf controlled will let any phone be used that supports dtmf most mobiles do and so do tone dialing type telephones.
MCU controlled definatly.... i wouldnt risk the use of the car pc during this tracking it may fail or crash or just run out of power.... ofcourse a log of lon and lat could be written to a text file for later viewing if needed.
As for cost like you say if you have an old mobile phone lying around that be perfect for the dtmf side of things as for actually trasnmitting hte NMEA data it would need to be capable of GPRS for web update or SMS compatible for text update or you could get the car pc to speak it to you for normal voice updates alough i would prefer option 1 or 2.