it is a long and tough drive. we were going to do thi over new years since we are spending christmas in oaxaca and have aplace to stay in ixtapa, but we decided against it. you pretty much need to drive to mexico city and then take it from there to ixtapa in order to take the good roads, otherwise it might be shorter but it is not worth it.
Have you already made the trip? I once drove (Ok, I was the passanger most of the way) from Huajuapan de Leon, Oaxaca to Ixtapa. Hwy 15, I believe it is, takes you out to the coast and then you head north on Hwy 200. I wouldn't trade the experience for the world -- the views were breathtaking in the mountains. But yes, you want to be with a good, sane driver. My friend and I actually drove from Napa, California to Oaxaca and back. Happy to share from my experience if you have any questions. Best . . .