A $9000 100,000 mile Mach 1 that is stock and never (or rarely) been beat on, and is in good shape (as the OP said it was) is not at all a bad deal. You could spend a couple few thousand dollars more and get one with less miles, but it could easily be in worse shape or have been driven much harder.
And miles really don't mean much these days. LOTS of people have gone over 200k on the stock Mustang motor. I have a buddy with a 04 Mach 1 that daily drove it for over 150,000 miles with full bolt ons, 150 shot, gears, built transmission and 3800 stall. He made over 1000 passes at the drag strip, on top of the 150,000 miles, before he finally spun a bearing (on the stock bottom end). That is a testament to just how reliable these cars are, even with some miles on them.
OP, if it is in really good cosmetic shape, mostly stock (indicating it hasn't been drive but so hard), and the maintenance has been kept up with it, I'd gladly give $9000 for a Mach. Clean ones are becoming harder and harder to come across.