Aquatek has a 120G (?) FS with tank, stand, canopy, plumbing for $350 I think. I needs to be scrubbed, but it is a good price.
If you buy used, check over the seams very closely. I usually tell people to buy the tank new b/c no matter how nice your equipment is, and how clean your water is, if you spring a leak, its all over.
Its like my father always told me about tires for your car - don't buy cheap. Why? B/c the rubber is the last thing between you and the road so if the tires fail, its not a good thing.
Yes, its more expensive to buy a new tank, but its worth it in IMO.