If it's exactly what you want and 17k for it is worth it to you, then i'd buy it!
I agree! $17k might be "a lot" for a Mach 1, but look at what other used crap out there goes for $17k these days. If a Mach is what you really want, go for it! Those low miles, original owner, and if its as good as it sounds, I'd say it sounds like a great car.