I'd get the airbrush for your vehicles but 28mm people? I'd leave that alone with the airbrush. Practice shading your fleshtones. I'll skip the teaser pic this time and just give you the straight dope.
Here's my tutorial on how to get better. It's called "how to paint naked women" and it does contain miniature nudity, so FYI.
http://skankgame.com/Painting101.htmlBasically, what I'm outlining there is to over-train yourself to do detailed shading with a brush, and the effect will probably be close to that "airbrushed" look without all the endless wrangling and clean-up that an airbrush requires.
Well, maybe one more teaser pic so you can see what I mean: