Hmmm, never seen an herb terr. What types were they suggesting? Some herbs I think would suffer from excessive humidity if you went with a more closed system, also heat build up could be a problem as most herb that come to mind need rather intense light. I'll look forward to seeing what you go with and how it turns out.

Media-wise, I think you might need something a bit more 'gritty' -- straight potting soil might compact too much for good root aeration.