There are factory built metal fireplaces that are NOT zero clearance. Some must be built into a masonary structure to meet code.
The minor cracking of your firebox bricking is probably normal. If it gets worse, these bricks (refactory) are usually a stand size and can be replaced. Purchase them at a masonry supply outlet.
As to the life, as long as you keep rain from getting down the chimney, and don't build too large of a fire, there is no reason they shouldn't last 30 - 40 years.