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So over the years I've gone from hand painting models to airbrushing. I've really had good results on my tyranids with the airbrush lately but I want to try some of the stencils I see on mechanized vehicles. I think eldar stuff lends itself the best to this but I don't know what codex to go for? Craft world or the standard codex? Are they interchangeable?

Eldar Craftworlds is the standard codex for eldar, Its all you need unless you wanted to run forgeworld units.

Well, Codex: Craftworld is the standard Eldar codex. And that's all you really need to do the army as you described. You would only need a different book if you wanted to paint a titan or any FW unit in general.

Craftworld is the standard 7th edition Eldar codex.

Also, if you want to do a mechanized army specifically to airbrush some stencils I would recommend going IG, SM or Ork. They have the most vehicles and arguably the best looking ones. There is something to be said about a well painted Kan Wall (As useless as it is)

https://www.forgeworld.co.uk/en-NZ/Imperial-Armour-The-Doom-of-Mymeara

and

https://www.games-workshop.com/en-US/Codex-Craftrworlds-ENG-IHP

With those you can make a mechanized list.

Suggest you look at the get started box sets for your core army since they come with the bikes and tanks you will need.

If you want to use stencils, keep in mind that most eldar vehicles don't have a flat hull.

I collect harlequins, and airbrush diamonds on the canopy of my bikes and skimmers. I have stencils from death ray designs (http://deathraydesigns.com/product/harlequin-airbrush-stencils/), and even if they're made of a flexible plastic, I just can't keep them flat on the hull of my skimmers (it would however be easy to use on a rhino).
I know some other companies make stencils, some are self adhesive which is probably better for curved surfaces. On Fall out hobbies' website, they have pictures of eldar skimmers that have been airbrushed with their stencils, so it looks like it's an option (but I've never tried any of their products myself).

Unless you meant painter's tape stencils that you make yourself (which is what I do for my harlequins), in which case it works like a charm.