There is something to be said about smart planning and not painting yourself into a corner. However, this building isn't rocket science. So, I decided to plunge ahead with two decisions that will impact my future sequence of operations.
First, I bought a piece of 1/2" thick Masonite that I will use as the base. As you have probably seen, the building has several floors and slopes along the length of it. While in real life it is paved along a grade so that all you see is a nice gradual angle, I am building this model right as a stand-alone building so I don't want to pave it or build it into any permanent scenery. So, I need a foundation that will look decent, support the two elevations of the building, be lightweight, and perhaps make it easier to carry the building around when I need to move it. Since it it isn't part of the model, I am not going overboard with the appearance. I will paint it gray to represent concrete and that is it.
I also plan to install little wooden blocks inside to loosely secure it from sliding. If I make them too tight a fit, and binding could damage the model. Of course, once the roof is installed gluing them will be more difficult so I started working today by cutting little pieces of wood and gluing them to the base.
I also took a giant Indiana Jones leap of faith forward and glued one of the roofs onto the building. The area of the building that I secured only has two windows and I think I can get them easily from underneath if I need to. The roof over the railroad loading doors will be loose for now until I decide how I will install the interior. The third roof, over the side with all the windows, I may glue now.