What country and state are you in? That will affect whether you need a fireplace. I like freestanding bookcase/storage units, they must be attached to the wall if tall and not deep. If getting custom built get at least 3quotes. Finally, there are DIY shelves available at hardware outlets and stores like Ikea.

Some handypeople may be able to do it. But you would want to find one who had handled a similar job before.

any gas log fireplace should have very specific instructions for safe installation with measurements for distance from back and side walls, height above floor and height of any mantlepiece above the front of the heater and details of the correct method to enclose the flue..first check these instructions and then decorative finishes will be much easier to decide...if you haven't chosen your heater yet all the instructions should be available on line and this will also help decide the best heater for the space available

depending on the size of the room i would add a wall of high and low, closed and open bookshelves on both side of the fireplace but difficult to be specific without a plan showing size of the room and location of doors, windows and names of adjoining spaces