If a DSLR proves to be too expensive, you could try a superzoom point and shoot such as the canon SX series. Until recently I shot with the S2IS which was one of the early generations of the superzoom. They are perfectly capable of taking excellent aviation photos like this one:
http://farm8.staticflickr.com/7273/7...857104b1_z.jpg
photo
I would strongly recommend a camera with a viewfinder and not just a large screen, since it is much easier to use the viewfinder than the screen in bright daylight. Something like the Canon SX40 IS which is about $300 on Amazon right now.