As to level there are a number of tests available - just search for placement test efl.
There is one here
Language Link. .Test
Resources - It depends on level and how you want to go about it. there are series of books like Market leader or In Company for Business English and they both have resource packs/supplementary material. There is a fair amount online too. It's easy enough to find things like sample business letters etc. Just search for what you want.
- For Teachers