Looking ahead rebuild motor and tran, how much where which one how big?

My buddy blew his truck up and they said they could not rebuild it. I was wondering when would one go about rebuilding a mustang? Can I wait until its broken before i fix it? What kit and who would I go to? Also any idea how much a motor and trans build run? I would like to get some more power but nothing crazy that will kill my gas mileage. Maybe keep it between 15 city 20 highway? I want to keep this car as a DD and get something big later on like a used 05 Z06 or a viper. My driving is habbits are the following: random 2-15 mile trips in the city, 9-40 and sometimes a 100+ mile trip on the high way 55-70 mph, so basically i drive normal types of stuff. What do you guys think?