I was literately in the exact same position as you a week or so ago. I was dying to find a Mach 1, but all I could find in the $10K range was stuff that was beat hard like that one you posted. I could have had nicer, but they were pushing $14K+, and I didn't want to spend that much. Around that time is when I stumbled onto my Cobra, it is much cleaner then most of the Mach 1's I saw, and has less mileage. Course mine has a SRA swap so the IRS strength issues aren't a problem for me.
If you can afford to get a nicer car, do it. I've had tons of beat cars in the past and it is a lot easier to start with a canvas that isn't all hacked up. Don't let the low buy in sucker you. You will pay for it somehow in the end. I wasn't willing to pony up the extra cash, and I still was able to find a car I am happy with. It just isn't a Mach 1.