That's how I started a few years back. Started on the wife's car and moved up from there. Even though I was new with a machine, I went for the Flex and found it very easy to use. I was afraid of doing damage as well, but quickly found that I wasn't doing enough to correct let alone do damage. I'm sure you've done your homework, read a bunch of stuff, watched videos, etc...So you know to do a test spot and to go with the least aggressive approach and move up from there to get the desired results. You'll be fine.
I'm assuming you will be working on the mustang for the 3 step, I'd use Pinnacle on that. It's made for red, black, and yellow and you will get great results.