Blank Canvas (How to use the space best - and the windows?)

If I were you, I would consider adding a window tint to the windows so you couldn't really see in from the outside.  That way, you could build the layout across the front of the window wherever you feel is convenient.  Also, this gives an added sense of security/privacy and will help to keep the building cool during the summer months with the sun beaming down through all of the windows.

I think it's a great space for the layout.  All of the natural light will make it a nice place to work and enjoy what you will build.

I see some white stuff on the ground, but no pink stuff in the walls. I think that could lead to a moisture problem down the track.

It's all been insulated since the photo.

Tinted window film is a good idea - perhaps only in the bottom sashes of all the windows.

Alternatively, you could remove the inserts in the bottom sashes and cut thin (1/4") plywood to size and painted white on both sides and install into the inside of the bottom sashes.

Either way would allow you to raise the layout height to halfway.

I'd pay little attention to the windows:

I agree with tinting the glass. You might even want to black out the lower section of the windows (maybe the door, too) and run your backdrop panels up to the top of the lower section. Make them removable so you can open the windows for ventilation, if necessary. You have a good space to work with.

I would suggest an around the room layout, with a lift bridge or drop down panel of some type at the doorway.

My layout is only 22" off the ground. Although this is low (I have a hip wall and had to place the layout below the wall-ceiling connection), it is fine for operation. I have a few chairs in the room, and will sit down when operating the yards and switching. Then walk around the room when running trains.

True, when working on wiring under the table, it is hard on the body. I am 63 and can still work under the table OK. Use pillows for the head. Others have used a trolley on wheels, however I do not have enough space in the center to do this. I make sure all my wire connections can be brought to the front of the layout, where you can work on the terminal strips, etc while sitting comfortably in a chair. This requires more wire length, however it means I can install and maintain the wiring in comfort. (for example, the AIU sits under the table, and can be unhooked and brought out in front of the table, so you can work on the wiring while sitting with the AIU on your lap while sitting in a chair. Quite comfortable. 

My layout is twice around the room, (11ft x 22ft) which gives me a longer main line. That means you would still be above the window sill, so as mentioned above, you can ignore the window blockage, or cover the lower window panel (or two).  I have included some pictures to show my table height compared to a standard chair.

I added the second level later, after the first level was completed. I kept adding and expanding, so the early layouts were smaller, which allowed me to get things up and running rather quickly. The second level is cantilevered off the wall, with metal L-brackets every 6 ft. The ugly trackwork where the two levels cross each other is hidden in the closet (which is along the end of the layout, behind the TV and is 11ft wide)

Hello. I think it is a great idea to use the space. I agree with a higher layout too. I am asking a question more than supplying an opinion. Does not sunlight fade the colors on the trains, scenery and buildings with long exposure? I would be concerned about that. If that is true, than certainly look into window tint that supplies a high level of UV protection to keep your trains looking good.
Donald

I'd probably blank-off at least some of those windows with easily removable insulated panels.

Your space has wonderful possibilities and without much expense. Based on my experience I would assuredly use an around-the-walls track-plan because this permits the broadest curves and allows all viewing to be of the inside of curves which look more gentle in the mind's eye than the same curve viewed from the outside. I'd utilize the corners for sidings:

Even a small corner can be used to add interest, as with this Lionel working pump-jack:

That is the space left outside an O72 curve.

Build the benchwork with vertical uprights for a backdrop.

That is a nice space you have; it's about 4 feet longer than my own.  I don't think I have anything to add that hasn't already been said, but I'll give my agreement to many points.  Yes, you defiantly will be able to get a lot more layout around the walls with a lift out/up/down bridge in front of your door.  I do agree it would be good to tint the windows because the sun can fade certain surfaces.  I think the windows will dictate the highest you should go with your benchwork.  If you do it like Lew showed with his attic layout, you can easily get to the window latch and raise the lower half of the window.  I like a backdrop behind my layout to take away distractions, but Lew did a nice job leaving his windows not blocked by anything.  Either way.  Yes, you could build the layout at windowsill height, but you are wise to realize that may be a bad idea.  I can assure you as you move into your 40s, you will like that less and less.  At 64 myself, I wouldn't even consider it. 

You have a plan for heating and cooling, so that major item is covered.  I see electrical outlets are being added, probably have been added since you insulated as well.  Are you adding more lighting too?  It would be good to have the layout wiring on a single power switch or breaker.  (I do not have that myself )  I think you have really thought it out well and are on your way to having an enjoyable layout space.  I'm looking forward to seeing what you do with it!!!

Most everyone has addressed the main points but most import is tint like Mark state UV light will fad most paints and will add security and privacy but remember at night with the room lite up the whole world can see into it.. I would suggest window tinting and some type of window shades or curtains for privacy when you are that in the evenings. Or black out the lower widows or put a backdrop around. To hide the layout. Along the lines of lighting I would consider LED track lighting on dimmers also make it sectional to highlight areas of the layout and good lighting over the entire room when working.  

You may want to have more than one circuit going to the room. Two or three breakers would allow you to have lighting, a TV and a computer on one circuit, transformers for track on a second circuit and accessories on a 3rd circuit. I only have two circuits on my layout, however with LED track lights, I am able to manage. However I have my track power and lights on one circuit, which is pretty heavily loaded. I usually only run one train at a time, however I have designed the layout to run 3 trains, which if I ever do this, may cause issues.

You can never have enough power in a train room.