Owning a D70, I can certainly recommend that one; even more now that it has been "replaced" with the D70s. I don't know the D50's specs but I would guess that sooner or later, you'll regret not having selected the D70s... If the price difference is not too big that is...
80-400 will give you effectively 120-600 (not really, but that's the effect, due to the smaller sensor in DSLR camera's). However, I wouldn't really trust a brand like Tokina for the high-quality needs of aviation photography. I know this is purely a personal opinion, and that lens may be extremely good (just try a google search), but you should be able to get a good second-hand Nikon lens for not too much money (I bought an old 75-300mm recently for under $300).
Especially if you are not contemplating going for better than F4.5-5.6 maximum aperture (which is OK
if your pockets aren't very deep) I'd recommend to go for the quality that a Nikon gives (once again, personal opinion, but I've used "cheap" lenses and keep going back to Nikon...)