>steampunk airship playset
Believe it or not, but I found an old SuperSoaker squirt gun at the swap shop that has all the convoluted curves and swoops a steamship vehicle would need. And it has clear-purple cutouts for windows! I'm adapting it as a Martian battleship for a John Carter model. Old Nerf guns might work too.
For that matter, were there any toys for the John Carter flop? (I enjoyed the movie, BTW, and so did my sweetie.)
You should be able to find cheap GOLDEN COMPASS stuff too. The vehicles are steampunky.
>Any suggestions on figures to keep an eye out for that would easily work as a base for my steampunk airship crew?
There are lots of cheap Star Wars figures you can buy on eBay that come direct from China. They come in batches of five, like five identical AT-AT commanders for $9.99. There are LOTS, so you can probably find good candidates.
>Any advice on good steampunk fittings and accessories for 3 3/4 scale?
Wander a hardware store or Home Depot. Various electrical assembly kits should provide what you need.
Also try to find obsolete microwaves or other appliances and rip them apart. All kinds of wires and coils and what-nots fall out.
That should get you started.