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.

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 think you should stay with a simple pattern for draperies. Possibly a pattern incorporating the white trim and warm yellow walls. This would be soft and understated and something that you wouldn't get tired of easily. Other colors could be introduced with accessories.

cool --but you can see the entire floor has been raised--do you have the ceiling height? a closet 'step up' would be less of a structural commitment, OR a buildup under builtin furniture--like bookcases flanking a fireplace, also some great ideas on HOUZZ i'd never have thought of. sorry i'm not tech-ie enough to paste the site:) of course if you go 'structural' --all walls and ceilings are fair game as well. remember that ease of access [or lack thereof] can affect stealth mode greatly--as in your pic: every time you step onto the 'box' in your pic you would hear a 'different' sound.