I guess you are not talking of a compact camera because they have a fixed lense, i.e., you cannot change them.
Since you will probably not get too close to the airplanes you need a lense with a high focal length, a telephoto (fixed) or zoom (variable) lense. Now there are two caveats with these. 1) The cheap ones tend to be iffy on the light circumstances. A good lense will be light intensive, but that makes it expensive. 2) Even small vibrations wil cause the picture to be blurred because the vibration is "magnified." [I don't know how to explain this better.] Thus, a tripod is a must.
Lenses tend to be the most expensive part of camera equipment. The better the light intensity the more expensive the lens gets. It also depends on which lense you get. Sigma is cheaper than Zeiss. One way of getting nice stuff cheaper is by buying it second-hand. See if there is a good second-hand photo shop in EDI
(you'll know it's a good one when the photo nerds hang out there), go there, and look around.
For the beginning you might wanna get a zoom lense as it gives you more flexibility, close-up - wide-angle. Something in the range of 300mm - 100mm is nice. If you wanna go spotting at EDI
for instance, ask the local spotters what they use. They'll be able to tell you whether a 300mm is not enough or too much.
Maybe here you get a feeling for what's out there ... http://www.sobotka.at/kamera.html