Poke around on craigslist for one. I have seen many 65-68 mustangs for under 10 grand that were restored maybe 5 or 10 years ago and the owner is moving on, or they replaced the drivetrain (engine and transmission) and the exterior is alright but not perfect. I would look around there and wait till I see the perfect one and go for it
You're looking at anywhere from 5-35k depending in condition and if all the numbers match (meaning everything is original). And if it's reliable, well that really depends on the type of people who owned it before, and it depends if they baby'ed it or really pushed it to its limits. And yes they can go pretty fast.
A very cool car.
But facts are if you're looking for reliability get a Toyota.
A 1968 mustang fastback shelby (My dream car) is for a select few who appreciate the beauty of it.
It doesn't handle like a newer car, it's loud and noisy inside and out.
The cost is about $250,000 in mint condition if babied since the day it was made.
I'm sure you could find a half reasonable good one for $60,000+ or a fixer upper for less.
They do make clones of these cars.