Advice on good business english course/reference books

I'm currently planning to set up business as a freelance english teacher proposing my services to companies. I worked for CEL in several CCI in France about 10 years ago and am therefore somewhat out-of-date with new methods and teaching materials.

I would like to buy a basic stock of books to begin with but my budget is rather limited.

Could anyone give me some constructive advice on what to invest in ? I will need some decent listening material and also some indicators for initial testing (levels).

Thank you for any advice you can offer.