honestly if the motor was well taken care of then it should be fine. these motors take a ton of abuse and smile about it the next day. that being said, you will need to monitor what the car is doing once you start modding it. in stock form the car can pretty much take anything you can do to it, but once you start running more boost the car may or may not care for it. every one of them is different so you have to scan it to make sure everything is healthy.
now, as far as you blowing up the motor and then having to find another grand prix, its much cheaper to replace the motor. just dont look towards the dealership when this time comes because they will rape you, even if you work there. im surprised you sprung for a transmission from them even. i am also a huge fan of the dark green (i had one too) on these cars and its not something you see everyday like the other colors. therefore, id hope you would decide to keep the one you currently have.
the next step for you is to decide if you want to go shade tree mechanic on this stuff and tackle it yourself or go the route you said and have someone else install it. doing it yourself youll not only save a stupid amount of money, but youll also learn about the car during the process. the knowledge you pick up may help you to fix it one day.
hopefully this helps you decide on what you want to do.
