Okay. im going to answer this in order:
1. Mustangs are NOT reliable they break down when they get old (over 100,000)
2. Reasonable price: $4000 - $10,000 (IDK if you want v6 of GT)
3. It should be less than 120,000 miles. Older than that is old and not worth it.
Mustangs are high in insurance and gas.
You should buy a Japanese car like a honda civic si. these are sporty and reliable and good on gas. American cars aren't the best.
If you are looking to get a car as cheep as possible you should try the GM College Discount Program. Any current college students or those who have graduated within the last two years qualify. If you are interested in learning more, check out http://bit.ly/mgpbFC or stop by the Facebook page http://on.fb.me/iFAEEk. Hope this helps!
in personal opinion i prefer foreign cars such as toyota and general a car will be better suited if it is a 2000 and up... as far as mustangs go they are good cars and pretty reliable up to 250 to 300 thousand miles tops. Hope my opinion helps, good luck with your car.