How many people will be occupying the building, what kind of equipment is running in it, and what kind of lighting?

Without knowing what kind of roof and how many windows you have it is hard to say for sure what will happen without modeling the building. You will have to open some windows to allow natural ventilation. Forced ventilation is worth every penny if you can find a way to do it economically. The cheapest way would be to find some used squirrel cage blowers. You can find them cheap from a local HVAC contractor and wire them in close to a window.

Without forced ventilation it is going to get hot and humid in the building. People and lighting generate heat, so do computers and any other electronics. Sunlight also heats up buildings quite a bit. The advantage to concrete is that it will take a long time to saturate it with heat. Concrete is very heavy and has a decent capacity for heat. This means that the walls of your building act as a buffer zone, if they are solid concrete. If it is just a block wall this isn't as valid.

It probably won't exceed 100 F in that building. With ventilation of about 5000 CFM you would knock that down to about 90 F. I would have to do calculations to be sure though.