Some semi-random thoughts from an XSi owner…
I’d sooner buy a new Rebel than a used pro body for the same price. Pro bodies tend to be used heavily, and often by people who see the camera as a lumpy black tool, rather than an exquisitely engineered tour de force of fragile technology.
The XSi on the eBay listing linked above is a tricky one - it only comes with one real lens and two adapter doo-dads that screw onto the front filter threads, and the extra battery is a no-name replacement, as is the flash. These things may be fine, but more likely, they’re junk.
I bought an XSi in May or so and made a major lens upgrade - instead of a basic kit lens, I got an “L” series 24-105 zoom. It’s given me some stunning images. When you get everything just right, the results are almost 3-D. (At least I think so, but I can’t actually see in 3-D anyway.) You’ll have to wrestle with your savings account over this - IIRC, it’s a roughly $1300 lens, but if you can afford it and don’t mind a somewhat heavy “walking around” lens, you will not regret it.
The crop factor just means that the image sensor is smaller than a standard frame of 35mm film. In the case of most non-professional Canons, it’s a 1.6 factor. Just multiply an EF lens’ focal length by that to find the equivalent. I think the EF-S lenses are labeled with the crop factor already factored in. This all comes to mean that my 24-105 mm zoom acts like a 38-168 mm. Great for getting some extra tele “reach” but it messes up your day if you want extreme wide-angle.
You do need to be aware that you can not put an EF-S lens on a pro body. The rear element of the lens sticks out beyond the mounting flange and will be hit by the mirror. I believe Canon has designed the mounts to make this impossible. You can, however, put an EF lens onto a pro body or a Rebel body. This was another factor in my decision to upgrade the lens - if I step up to a pro body down the road, I didn’t want to have to re-buy all of my lenses.
Going back to an SLR format camera has really changed how I take pictures. Over many, many years, I went from a pocket camera that used 110 cartridge film, then a basic 35mm SLR, a more advanced SLR, an autofocus SLR, then a digital pocket camera. While I was getting more and more fed up with the digicam’s performance or lack thereof, dSLRs were coming down in price, so I made the change. No more waiting five seconds for the thing to turn on and then waiting a few more seconds for it to focus and take a picture. No more sucking a fresh set of batteries dead in 30 minutes.
The dSLR is practically instant. The limitation is really more your own reflexes - you can turn the thing on and take a picture in about .2 seconds. And its power management is amazing - I’ve taken close to 5,000 pictures and have had to charge the battery three times so far. Just turn it on and start shooting. And yes, you will want to shoot anything that sits still long enough for you to aim. Or not - the sports mode lets you use the camera almost like a video camera. Just hold the shutter button and follow the action.
One last thought. The XSi does have a characteristic sort of “squeak” to its shutter noise. That’s been the one wierd thing about it, compared to a pro body. Most pro bodies make a more subdued clack. You probably wouldn’t want to shoot a wedding with an XSi because of this sound.