There is a lot of advice here to take in. I can tell you that I've done high-end RE photography for several years now on homes from $1 million - $50 million USD. I did it for a long time with a 50D and Tokina 12-24 crop lens. It's focal view was that of a 19mm on the 50D. I would shoot straight on to avoid post processing distortion if I could, but still had to correct for barrel distortion. You can correct for keystoning in post, but it just stretches everything out. Sometimes you have to do that to "see" over objects in the room. I've done it plenty (shame on me).
I just upgraded to a 5DmkII FF and 17mm TS-E. It has enough view to get the tightest of rooms. I think the 14mm would be too wide. I find that the 17mm TS-E distorts the view more than I sometimes like. The plus is it doesn't have the barrel distortion of my Tokina and I don't have to correct keystoning.
I use HDR techniques on a tripod at twilight when the ambient light of the house can take over. I bring a box of light bulbs and replace any compact fluorescent bulbs so my light balance is consistent. I lock my white balance down to about 3000K, F11 and use Exposure Blending in Photomatix Pro for a very natural, realistic view of the space. I have never had a client say anything other than they love the shots. They don't ask HOW I process the photos, they just love the results. Better than them coming in with an on-camera flash and snapping away.
Setting up lights would take too long and take away from what the lighting designer intended (on higher-end homes). I can shoot an average house in 45 minutes to an hour, but spend 2-3 hours in post. Some bigger places may take multiple trips because twilight only lasts about 45 minutes.
If you are not doing this full-time, on high-end properties, you would probably get the most benefit from the 17-40 or 16-35, as you could use it for more... enjoyable shoots. I've heard good things about both but don't own either. I'm surprised to hear that even the 16-35 is soft in the corners (I've read that about the 17-40). How much more than the 17-40? I would rent both and see if one out-performs the other (I will).