Hi,
The basic setup for the device is an arduino (uno or mega), a gps shield and a gprs shield.
I have experience with both. The fastest way is by simply stacking up them, and coding the program.
The coding time is estimated in no more than 5 five days including all the testings.
At this stage we will have the functional prototype, a little space wasting but working.
The next step, is the design of a custom pcb with only the necessary components.
I use Eagle as EDA, and the OSH Park community service for the pcbs manufacturing.
I estimate that the size will be no bigger than and arduino UNO board, but including de gps and the gprs modem.
The fabrication is the part that needs more time. Usually 2 weeks. So if you like the custom pcbs, you should know this.
I could make a little enclosure with my 3D printer.
Any doubt, please feel free to send me a question.
bye,
Pablo