The best way to see if you will be able to maneuver the tank there would be to build a PVC frame the size of the aquarium and move it down there like you would a real aquarium. This has worked for me during my moves it allows you to find obstacles you may not have thought about such as ceiling fans and light fixtures. Just make sure to add cross braces in the middle so the frame does not flex too much while moving.
The tank should fit through the door, but with out seeing the actual location it is hard to say if it will fit down the stairs or not. The sugestion on the last post is a good one if you are concerned about it not making it.
Right on if you need any pointers or advice you are in the right place lots of people have big plywood tanks here. Much cheaper then acrylic if you are willing to put some effort into it