I would suggest picking one or two industries at most, as you will need to do research and come up with interesting articles monthly. If you are write for many industries you might not be able to come up with quality content each month. That would put your customer`s newsletter at the bottom of the pile.
Since many articles could be timeless you can offer them as a portfolio so the customers can customize their newsletter from the list of articles.. Maybe even have a few templates for the newsletter layout to choose from. For those customers on the net, a website where they can go online and check off the items they want.