Jules, you need to get an excellent understanding of light, color, composition, post processing (compositing, HDR, color handling, focus stacking, layer blending, frequency separation, cloning, etc, etc, etc) before you consider going down this path. Your 18-140 will give you the width you need, and if not, you can generally do a stitched pano to get wider.

First image - horrible keystoning. poor control over light, not sure what you are wanting a prospective buyer to look at that says "buy me."

Second image - wrong time of day, too much contrast, the hour after sunrise and the hour before sunset would avoid your blown out sky and concrete driveway. Don't need to show the side of the house, it is unattractive. Slight keystoning here is ok, but I would have corrected it.

Third image - window totally blown out, keystoning, barrel distortion correctable in Lightroom, Photoshop PT Lens with the proper lens profile. Doesn't really show the room to it's best advantage.

In smaller rooms you will need to use a wider lens, but carefully, to avoid keystoning, volume anamorphosis, severe extension distortion. Or you can use a longer lens, but you'd have to start being concerned with having adequate depth of field, so you'll have to learn how to do focus stacking. A tripod with a bowl platform (often used in video) and a level will be indispensable.

Take a look at this guy's portfolio. I mentored him a few years ago and his work is really excellent now. You probably don't have the clientele, especially if you are doing MLS RE Agents, but you can certainly learn from his approach.

http://ronnachtwey.com/bas/beforeafter-residential/

FYI, he was a Canon 5d Mk III shooter until he found he needed a 14-24mm, at which point he bought a Nikon D800. A comparably wide lens on your camera would be an 8-16mm or a 10-20mm. I think the 12-24 or the 14-24 may not be wide enough for those difficult, tight spaces. But if you understand it and use it appropriately it should be your least used lens.

I use my 12-24 f4 AFS although not necessarily for real estate photography. At 12mm (18mm if using full frame) it could be very useful for many wide angle shots. The lens is sharp and professional grade.
I have two observations that I would like to share with you. The first one is that one flash alone will not be enough and the other is that for a more realistic look in many of your interior shots you will have to go HDR (high dynamic range) photography. Your last shot, even when I do not know what was outside is a good example. High dynamic range photographs will show properly exposed what is outside.
Useless to say that a good tripod is a must. Flash modifiers is something that you also should consider.