Ifrat and Rayaira need to keep their big heads to themselves and quit destroying the kids thread to where it gets closed.. He asked a simple question, and the two of you had to come in and start running your mouths, Grow up and leave it to mypace.
Op, all the other advice is perfect. As for the upstairs part, you should be fine as long as you keep it against a wall where there is good structural bracing and whatnot.. Any well built house/apartment should definitely be able to hold a 125 gallon setup. As for the water changes, I would for sure bump that up atleast to the 20 gallons a week until you can upgrade, more if you can.. Good luck to you and your fish.