A good glass tank will last for ages! The sealant might get old and leak a bit (after a 100 years or so) so you may need to empty the tank and remake the silicone sealant, but you'll notice for sure when this happens.
If you want to be certain to get a tank that will last, you could buy a completely new one from any pet store
Second-hand tanks are cheaper, but they may have been used quite long already, so the risk of leaking might be bigger.
Tell your parents that the floor is the last thing to worry about d: