Santana makes good tandems, $500 for a 15 year old tandem is not out of line if it is in good shape. If the owner will let you pull the seat posts and look inside the frame for rust. You will most likely find some light rust after 15 years, light rust is ok.
Check the drivetrain and wheels are they in good shape, tandems are very hard on drivetrains and wheels.
Then check the fit, it has to fit both of you.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle