Good choice, I love those books (must dig them out again soon!), and I have thought about collecting figures for them on and off before.
I may be misremembering as it's been a while, but I think any sort of high-Victorian, VSF or pulpy figures would do. Bastable remains fairly constant throughout the adventures, so a stiff backed mustachioed officer type would do for him. Many of the other characters from the wider Cornelius-verse seem to change their appearance from story to story, even down to skin tone in some instances.
Supporting characters would be mercenaries, air pirates and so on...?
I would recommend having a trawl through Artizan Designs' pulp figures, and of course Pulp Figures themselves, especially for air pirates. Copplestone will have some good ones too.
Again, I may be remembering wrongly, but the books take in a big chunk of time from circa 1900 (or was it 1920-ish...?) to about 1945, and encompass some alternate realities as well, so technology, weaponry and uniform styles etc are open to whatever floats your boat... Anything 1930s Back of Beyond style would probably work...
Basically, what I'm saying is a mix and match approach would probably be the best.
As for airship fleets, is Spartan Games still running? I seem to remember them having something similar to this...