If you are going to get a long arm it is going to have to go up against the wall and on the 13' length. You will not have much room for anything else.. My craft room is 11' square and I have a 10' frame up on one side of it. It is a cramped room but it suits me fine...
Not having a closet is a downer. You won't have much area to store fabric - unless you store it under the longarm in boxes which won't be fun but it's doable.
I like color. My walls are a creamy color and I hate it. I went to the Dollar Tree and bought a bunch of wall stickers - the removable kind know what I mean? and I jazzed it up a bit. It helps but it isn't perfect.
You'll love your own sewing area no matter what you do with it :)