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

Not for me, I find the concrete floors too harsh, the one good thing about them when they go off trend easy to cover with something else.

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

I would get the installation instructions for the fireplace you’re planning on having. They are usually available online and will have information on the surrounding construction required. Builders often have a reluctance to do something new, but they are also the one that has to build and warranty the work. You need to negotiate a position that you’re both happy with. If this column is just plasterboard and is just a cosmetic design feature it shouldn’t be that expensive. If the heater is installed as supported from the construction above rather self supporting from the base it may be a reasonable cost.