You are paid as an active duty officer while at ODS. There is a small allowance for uniforms, but it's something like $400; you'll spend much more than this at the uniform shop.
Off-base liberty policies depend on the class. In general, liberty privs increase as you progress, unless your company or class do something stupid along the way.
Don't bring an iMac. Your room is kept to exacting standards during the day, which includes leaving nothing on your desk except for the books that are there when you arrive and a small family photo. Everything else must be stowed in your footlocker when you are not in your room. Unless you want to unplug, stow, then re-place, re-plug, etc. multiple times per day, it doesn't make sense to bring a non-laptop computer.