Its been a while since I looked at cameras but I would stick with Nikon or Canon since they are the major camera brands. Now I hear that sony are making some good cameras too but in my opinion the above two offer a range of cameras and lenses going back a while, hence you should be spoiled for choice even with 2nd hand items.
As for which camera as I said I haven't looked at cameras in a while but generally the more professional a camera is the harder it is to start getting good pictures from. For example my old D70 had a fully automatic mode that even deployed the flash for you whereas my D200 doesn't do that. So I suppose if you are just starting and don't really want to get too bogged down before you can use it, an ameature SLR would be better, for example a Nikon D90 (I use Nikon but someonw will surely com along and point you to the Canon equivalent).
One other thing to consider is size and weight, a D3000 is relatively compact and light, but then it will probabaly have less buttons which means more digging around the menus and would be slower than a semi-pro. Also note that the newer Nikon lenses use have what is called AF
-S focusing, this means that the lenses have an inbuilt focussing motor, the older ones don't have this. What this means for you is that with something like a D90 you can autofocus the older lenses, but with cameras like the smaller D50 you can't since they need the built in AF
-S motor. I don't think canon has this slight issue.
Now I see that I have wrote an essay without giving a direct suggestion but with a camera I suppose it depends on what you want it for. If you just want it for taking photos of vacations and stuff like that, then you aren't going to notice that the shutter cannot give you 5 frames per second but is limited to 3. Also the kit lenses that tend to come with the cameras have the AF
-S motor and you proabably aren't going to need to buy an older one if you just want snapshots. Usually people buy older lenses because you can get cheaper professional lenses, for example a new Nikkor 17-55mm f/2.8 will set you back 999 GBP.
And finally there is the price, how much do you want to spend?
You may also want to try and post this in the aviation photography forum since you are bound to get more attention there.
BTW My spellcheck isn't working so excuse any mistakes, hope this helps.
Lady Guinness is ready to fly...