Camera
The 40D was introduced in Q3 2007.
The 400D was introduced in Q1 2007.
The 1000D was introduced in Q3 2008.
All were built around the same basic 10.1MP sensor.
- The 40D sensor has improved microlenses over the 400D and outperforms it in terms of high ISO noise.
- The 40D records the sensor output in 14-bit raw files versus the 12-bit 400D.
- The 40D uses the DiGiC III vs. the 400D's DiGiC II in-camera processor.
- The 40D has a significantly faster burst frame rate: 6.5 fps to 3 fps and a deeper buffer that allows you to take more images before the camera gets bogged down waiting to write images to the memory card.
- 40D shutter lag is shorter at 59ms vs. 100ms.
- The 9-point 40D Auto Focus system is slightly faster than the 9-point 400D AF system.
- The 40D has a maximum shutter speed of 1/8000 second compared to the 400D's 1/4000 second.
- The 40D has Live View mode that allows for more precise manual focusing using a magnified image on the rear LCD when the camera is mounted on a tripod. The 400D does not feature Live View shooting.
- The 40D has 3" rear LCD vs. the 2.5" screen of the 400D.
- Maximum ISO is 3200 for the 40D versus 1600 for the 400D.
Add to that the magnesium alloy body vs. aluminum/polycarbonate, two
control wheels and a joystick vs. cross key buttons that require more
delving into menus to change settings, slightly larger viewfinder, a longer shutter life rating (100,000 vs. 50,000), and better weather sealing and it begins to be clear that the 40D surpasses the 400D in terms of performance and durability in practically every way.
The 1000D, while the newest of the three models, was introduced as a budget level introductory model one tier below the x00D model line and two tiers below the x0D model line. It is a mashup between the ingredients of the 450D, 400D, and even retains some of the limits of the 350D. 12-bit raw files, maximum ISO of 1600, slower burst rate and shallow buffer, a 90ms shutter lag, 1/4000second maximum shutter speed, a slower 7-point AF system, and the smaller 2.5" LCD are some of the areas where it lags behind the 40D. A DiGiC III processor, improved microlenses, a 100,000 cycle shutter rating, and Live View mode are areas where it matches the 40D.
If small size and weight are important to you, the 1000D is the lightest at 17.5 ounces (497g) compared to the 400D at 19.9 ounces (564g), and the 40D at 29.5 ounces (836g). It is slightly taller but not as wide or deep as the 400D while the 40D is about 3/4" taller and wider and about 1/2" deeper than the other two.
Lenses
Your lens choice in this range of lenses is probably going to be more about what suits your particular shooting style than a side-by-side comparison of their optical qualities. For your stated primary purpose of holiday/party shots in indoor low lighting none of the three you have listed are going to do very well. They are all three too long and too slow (their narrow maximum aperture forces slower shutter speeds) for that purpose. For indoors use on an APS-C sensor camera lenses with minimum focal length of 70mm or 80mm are going to be way too narrow to get everything you want into the frame. And a lens usable at f/2 will allow a shutter speed 1/4 as long as a lens that only opens up to f/4. With the limited maximum ISO of the cameras you are considering this is a key consideration!
In the extreme budget category consider something like the EF 50mm f/1.8 II that can be had just as cheaply as the three you have listed. If you insist on a zoom a used copy of the EF-S 18-55mm f/3.5-5.6 IS II can also be had in your price range. Avoid the pre-IS versions of this lens as their optics suffered from poor quality control. The EF 40mm f/2.8 is another inexpensive choice to consider.