Heya,
Just to further mix it up, or clear it up (clear as mud!), here's some more specific info:
I would question if you need a straight on dSLR or not. The strength of dSLR is actually in the AF system and lens selection (telephoto wise especially). There are some other strengths, but those are the dominating ones frankly. I would take a look at what you're looking to do, and get a system that is suited to it. dSLR can be made to suite nearly anything, but it always takes several bits of gear, and it always just keeps adding up. Or, you can get an all-in-one system that can grow if needed, but otherwise, has everything you need.
So, on that note, 18mm is what you're currently using on the wide end based on the lens you're talking about on APS-C sensor size. This is basically 28mm on a full frame, in terms of field of view. If you were to go to another smaller sensor like a 1/1.7, then 6.1mm would be the wide end to do the same thing. The Canon G16 for example, has this. And it's aperture at that wide of an angle is F1.8. So you get wide, fast aperture, and the ability to shoot long exposure and RAW if you want. And further, you can add flash if needed (has the same hotshoe as normal Canon). It has decent higher ISO performance for what you're looking to do, it will show some significant noise at very high ISO due to it's smaller sensor, but for what you're doing, you don't need glossy smooth glass like walls. A little grain/noise is probably totally fine for what you're doing.
The wider the angle, the more depth of field you'll get for aperture. So on a very wide lens, you can stop it down a little and get tons of depth of field. So for example, that little 6.1mm wide end of that G16, is F1.8 fast. You could stop it down to F2.8 which is still fast, prime fast, and get enough depth of field for what you're doing. In fact, 6.1mm at F2.8 on that sensor (G16) focused on something approximately 6 feet in front of it, will have a depth of field of 3 feet (near limit) to 195 feet (far limit). That's plenty for an interior. More than necessary even. So you get depth of field and wide aperture, to allow for lots of light coming in, and keeping everything in focus. Add in ISO, and you can get plenty of light with a slower shutter speed to effectively expose the interior of a building.
If you want to take it up a notch, you can add flash. Flash is probably the best way to effectively light up this stuff. But it requires you have a good working knowledge of exposure, because it will either be two exposures (ambient & flash), or just one exposure (all flash). The flash will eliminate the need for tripods, long exposure, high ISO, etc, and simply allow you to light up the interior in several places so that nothing is really dark or in shadow with strategic placement of speedlites. You can get inexpensive speedlites to do this. And you can even remote control their power levels, etc, from the camera, to make it even easier. I would use the Yongnuo 560 III system here, $50 per flash, with a TX-560 controller. Each flash has a built in wireless receiver, so you don't need anything else. And the TX-560 controls all of them, and you can change zoom, power level, etc, in groups if you want, from the controller on the camera. You just place a few 560 III's around the interior and turn them on. Go where you wish, flash and take images. You would need a camera still of course, that can use the flash system, you could use any dSLR, or you could use a compact like a G16 or even a mirrorless like an EOS-M. Image quality wise, all are over kill for what you're doing basically.
Also, if an interior is too closed in and you can't get wide enough to capture it all, you can always take several images at different perspectives and stitch them together (panorama style) to simulate a wider field of view. Software to do that is free (Microsoft ICE).
You really don't need much of anything, anything from a 20D to a 40D is going to be overkill. You can also just get a T3 or XS, XT, XTi, XSi, etc. All of them are going to be do what you want. An 18-55 lens is more than enough, though it may not be as wide as you would like. That's going to easily cost $200 to $350 depending on what you get. Or, you can just get a G16 for $300 and you're done (with wide aperture, high ISO, live view, RAW, flash control, etc; all in one). A cheap tripod will do the job from there.
Very best,
:: My 
