An interesting topic:
I noticed it was rather challenging to shoot down pilots who were using the initial beginner planes (these pilots had more experience than the usual Aspirant). This was mostly due to the fact that their plane speeds were so slow compared to mine, that I'd overtake them before I get a chance to shoot them in a dogfight.
So, after some curiosity, I started to fly the "noob planes" again. To my surprise, my K/D ratio did not go down -- in fact, it went up! My first game was 18/2; and I'd found it easy to get planes in my sights during a Dogfight, for some reason.
I'm sure I'm not the first to try it, but if you're a seasoned player, you should. It certainly makes things interesting.
As for which plane to buy first -- I honestly don't know. I really bought each plane randomly as I got the credits. I don't even own all of them, in fact.