I would paint the entire fireplace including the mantel, all trim and shelving white. I like the Whisper White, but you must bring the paint samples into the room to see for yourself. The sheen on all woodwork should be semi-gloss, easy cleaning and looks crisp. I agree with painting the walls and I would paint the part above the mantel the same color as the walls, Summer Hue is beautiful, but once again I have no idea how that would look in your room. The walls should be flat most brands make a fairly easy to clean flat. If the fireplace were the focal point of the room, it could look very cool and dramatic to paint it a very dark gray almost black. But at least in this picture it seems that the entertainment/shelving unit is the focal point. hmm .... better stop while I am ahead.

This Cabinet is $155 at Ikea. You could take the 4 items you had hung, and put them in a "shadow box", or individual deep frames to tie then together, to create I piece that looks more substantial on the wall, Then add a little ledge for photos, and you would even have room on the cabinet for the cool bottles that were on the floor, and that cool chest could definitely find a home as an end table, or entry table. You could also add a larger plant in a really nice pot in turquoise. The cut and paste renditions arent always completely accurate in scale.

A thick concrete wall can keep your house cool. Steel will make your house hot unless you insulated the walls well. Insulated walls don't work very well in a flood zone. When they get wet, you need to tear them apart. You will need concrete for the foundation most likely unless you are driving steel pylons. There is probably a combination of the two that will suit your needs best. It would be helpful to know what you want to achieve by using steel. There are advantages to both. Steel floors can be cantilevered further. It is easier to make bigger window openings. Floor and roof spans can be lighter and quicker to build with steel, but the roof will get hotter and therefore need good insulation and need to be well ventilated. These are just general assumptions of certain building practices so that you know what to ask people where you live. The practices and resources vary from place to place and building something for a specific purpose can change the benefits of one material or the other. My best guess is that you will probably want to use both.

I'd go for make it white; then right there, flop one nice little white overstuffed loveseat or something that fit, or one big whopping chair, and fit in a tiny table and light; treat that corner as it's very own micro-retreat, and the seating is to not face out into the room. Like little lounge spots put under stairs, make this one dedicated to the view; and make it minimal. Decorate the rest of the room as if the column and retreat bit isn't there.