I may be a little late on this thread but...
I have an 07 GT with remote start and the range is actually really good. Ive been able to start it through a brick wall while inside my work, and across the street from a few hundred feet away. You might be able to buy the OEM system from the dealer as a replacement, but i'm not sure if they would install it for you since you don't have the system in the first place. They would probably charge you an arm and a leg for it as well if they did. It would be cheaper to go with aftermarket and it would be easier to find someone to install it if you didnt want to do it yourself.
