The Canon EOS Rebel X should work reasonably well. I use a Canon EOS Rebel G which from my quick research on the web is a newer version of the Rebel X.
As an EOS camera, it should be able to use any of the variety of Canon EOS and EOS-compatible lenses, of which there is a pretty big variety. They can be somewhat pricey; if you're on a tight budget you might want to try eBay. I bought my primary lens, a Tokina 28-210mm lens, through there.
How much zoom you will need depends on where you want to shoot and thus how close to the airplanes you can get. I have found for myself that a 28-210mm lens works in most cases for me. More is always better as it will give you more flexibility, but will also cost more.
As far as film, it depends on what you want to do with your photos. If you are shooting for yourself, you can shoot on whatever you like. Prints are popular for beginners since most people are already familiar with dealing with them. Though don't think prints are just for beginners, many long time shooters I know shoot prints. From what I have heard, prints are also more forgiving than slides so you may want to work with those for a while.
If you are interested in getting into photo-trading, then slides are where you want to head. Specifically, Kodachrome is the preferred choice, ISO 25 or 64. Fuji Provia 100F is gaining popularity among photographers though I don't know how many collectors are interested in it yet. Kodachrome has proved itself to last 50 years, color shifts over time are common with Ektachrome and other older E-6 processing films. The newer E-6 films, by virtue of being newer, haven't proved their longevity yet.
For prints, you probably will want to avoid anything faster than ISO 100. My favorite print film, that I currently prefer to shoot with, is Fuji Superia Reala 100. Kodak Royal Gold 100 is also excellent; Royal Gold 200 isn't too bad if you need to shoot on poor weather days. You can also get reasonable results with Kodak Gold 100. Again if you're shooting prints for yourself, try different films and see what you like.
If you want to put your photos on Airliners.net, you'll need a good scanner. If you can afford it, a film scanner for scanning slides and negatives will probably give you the best results, although some flatbed scanners can do a good job of scanning slides and even prints.
David / SAN