OP, here's an idea for a starting point. Pick up a bank of used full length lockers, and add steel to the inside or outside to make them stronger. The panels and frames can be welded together, and the side panels are set up for anywhere from one to 5 units making shelving easy to set up. I have not set up the additional strength or fire proofing levels yet, but I will once we have moved out of NY to a moire permanent home.
I started with a bank of five lockers I picked up on Craig'slist for $100, and wound up with a single unit, and 2 double units by removing and relocating 2 dividing walls. Here's what I started with, and what I ended up with. The lockers are a good point to start from, and they are easy to set up as you see fit. A block of four lockers with a dividing wall on a split one set of doors would give a decent amount of storage with 2 sided access (at the door or one unit back from the door) to said shelves.
Here's a link to the thread on my school locker safe. http://www.firearmstalk.com/forums/f171/school-locker-safe-100562/