I'd like to know, will I be able to get all future updates and possibly third-party keys if I purchase the game now, in its early development phase?
Short answer: Yes.
Long answer: Yeah, I should make that clear somewhere. If you buy the game now you will get all the updates up to the final release for free via the downloads section. If/when the game goes on Steam/Desura, I'll do my best to convert/issue the relevant keys.
(What I mean by "up to the final release" - this is just wiggle room if I decide to make a paid expansion or "Airships 2: The Airshippening" one day. There are plans for neither, but I'm basically covering my ass here for the sake of my future self - hope that makes sense. :) )