I will need to make a celebration of Life cake. However, we don't know when this will happen. It could be 1 week, month, etc. I wanted to make the decorations ahead of time. Any suggestions on whether I should use modeling clay, fondant, gumpaste, etc. and the storage time of each.

Depends on what your making. Gumpaste will last forever as long as you dont break it. Store it at room temperature in a box that isnt clear. (light can fade the colors if it sits for long periods.)