I totally agree you can go as deep as you want into the tuning but it's not necessary, theTeam, Race Tuned cars you win are awesome just as they are, easy to win and gratifying to win as well. You could just stick with those cars (Like the VIS Integra you can get early on) and still keep up with the guys that like to tune/upgrade to the max.
You guys have convinced me, going to pick it up tonight after work. My wife will kill me if I become as addicted to this game as I did GT4 a few years ago. I can't wait for the "aren't you too old for this stuff" comment to come about
Prepare to get nothing done around the house However, letting her paint and design car decals might get you out of hot water.