i have done a similar project, so here's a few pointers..
1. making everything from scratch is a pain! i know how satisfying it is, but there are so many complex problems from the selection of modules to PCB layouts and casing design. and it may end up costing more..
2. if u still want the satisfaction of doing things your way, just buy the Telit module from sparkfun. it'll cost around $300 and you still would have to program it.
3. if you just want to get this thing over with, i recommend the globalsat TR-151. its small, cheap, and battery backed. as a matter of fact, i wish i had this when i did my project. if you want something flashy, use the Geo-something from sparkfun. it costs a little more, but i am personally infatuated with sparkfun, and they can send it right to your door.
4. connection is always a problem. GPRS is a great choice for the real time applications(think small dots moving across the screen). the price per byte data is the cheapest. however, you would need a server with a public IP for this to work. so yeah, unless you are willing to invest heavily on this, this solution is out. another choice is SMS / Text messaging. cheap enough to pay, easy enough to configure, and if you just want to know where it is whenever you query, it'll work.
5. the map is another problem. detailed topographical and street maps are available, but not for everywhere. granted, you could always connect it with Google Earth, but depending on your locations, the deatils are sparse.
there are so many other problems, but those are the major ones. you can make it if you put your mind to it, but it's such a large project. anyway, that's my input. if you need any help or discouraging words
just send me a message.
cheers