Just using google keyword external tool alone would not be enough. I explain why - the number of searches per month (local or global) can be huge, but it also mean that competition is high, so your chance of success is low. What you want is big number of searches and low competition, otherwise you won't have a chance to rank high with your keywords. The best way to ensure you have the right balance - keyword popularity and low competition is to run comparison - any keyword above 1000 searches per month and competing urls below 200,000 is the key. P.S. Let me know if you'd like to know how to check number of competing urls.
If you use the Google tool, it gives you synonyms as part of your results. You can look through those to get more ideas/keywords to put back into the search tool. Or, you can start searching yourself based on the catagories of your product.