As Irv said, if it has teflon gears it will wear out, or they will crack. If buying used take some jeans fabric and fresh needles and run it to see if it hickups.
An industrial or heavy duty older machine will be the ticket. Harder to learn on with a clutch motor but can run large needles nad handle heavy fabric.

Here is one on CL
http://denver.craigslist.org/for/5552761672.html

Also heard that the new Singer Heavy Duty machines are ok, but can't vouch.