What words should be avoided in the preview text of the email?

@John, I don't think the Preview (or pre-header) text is weighted any differently than the body content of the email, but may be weighted differently than subject line. Every Email Client filters differently (and most don't publish their rules), but subject line is almost always going to be weighted heavier than the body copy. So, yes, you should still avoid spam-heavy words in the Preview text, just as you would in the body copy. The best thing to do is pre-flight heavily, and if you see warnings or spam filter checks failing at specific email clients, try adding more text that doesn't contain spam heavy words, so the ratio of safe to spam-heavy words is less severe. Or you can also incorporate some of the spam-heavy words into images, which inboxes can't read. The downside is that your recipients with images turned off by default can't see the offer or call-to-action, but check your device reporting to see how much that matters. If you audience is primarily viewing your emails in inboxes like Gmail or Apple Mail, which download images automatically, then you're ok, If your audience is mostly in Outlook, the image route may not the best choice.

And, ultimately, you'll only know what works best for you by testing! A 50/50 split test would work great. Try one version with the spam-heavy words in text, even in the pre-header, and one with the words "hidden" in images. See if either affects your inbox placement, and then also compare your engagement rates. We've done this test with bulletproof buttons VS images. Bulletproof buttons fail the Postini spam filter check, so our inbox placement is at risk, but image buttons (which are our calls-to-action) aren't visible with images turned off. Surprisingly, we found that bulletproof buttons get us better engagement overall, even if we get snagged in some Postini spam filters.

You could even try a 50/50 split test with the spam-heavy words in the pre-header VS lower in the body copy. Share your results if you do! I'd love to know if that makes a difference.

Email subject line and preview are the first things controlled by a spam filter, therefore yes, the spam rules applied to all email, therefore also to the subject and preview.
I found very useful tis article by Hubspot:
http://blog.hubspot.com/blog/tabid/6307/bid/30684/The-Ultimate-List-of-Email-SPAM-Trigger-Words.aspx

and, obviously, the Return Path Blog (where there is a lot of discussion):
http://blog.returnpath.com/blog/return-path-2/can-i-get-a-list-of-spam-words-to-avoid
http://blog.returnpath.com/blog/julia-peavy/free-is-not-a-dirty-word-and-4-other-subject-line-findings