Before any work is done, one must figure out if the bricks you are attempting to remove are only a facing or an integral part of the structure? Before you start removing bricks, a word of caution, there exist a lot of weight above you, possibly 5 tons. Lowering your wood mantel may not be possible if clearance to combustibles become an issue. A way around this, might be to eliminate the wood mantel and install a blue stone one

Like stated above, you need to be sure this is just one layer of decorative brick with just drywall behind it. If it is one of those fireplaces that the chimney is brick and all, you have a bigger issue. If just decorative, you will need something like we used to remove our faux rock. We got a little pneumatic attachment for our aircompressor. It lookes like a large screwdriver or mini jackhammer. Starting at top, you chizzle away. This is very time consuming but can easily be done. There will be lots of dust and debris. When all you have left is the studs, you simply put in new drywall. Again, this is all easy, but since this is a fireplace I am not sure what safety issues are involved like if you need specific materials on wall because it is a fireplace. My regular firebox just had drywall and so does my zero clearance.

Also as stated above, there may be issues with what kind of mantle material you can safely use and at what distance from firebox. 12" is pretty standard but depends on mantle material.

We lowered our mantle but we also brought firebox down to floor level. Rebuilding it was not that difficult. We simply built the legs, built the mantle and hooked them all up.

The cost of redoing the brick is up to you. If you do the work it is just materials. But obviously hiring someone to do this will cost you. I would imagine mortaring brick is very time consuming because they are so little. But a seasoned professional can go pretty fast. But again, it will cost you.

Sounds like what you want to do is doable and can vary in price depending on who does the work.