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I am considering purchasing Dressler's Understanding Criminal Law. Is it okay to buy an older edition? The older versions are only a few dollars!

Are there any other crim law supplements that may be more helpful?

Older supplements are fine. IMO, not much would change in Crim Law

The law won't be that different between new and older editions. But if you have Dressler's casebook, the newer edition of his supplement is basically keyed to that and is worth the extra money.