The keyword someone is converting on is the big question in any business. And it is also dependend on the vendor. If you are trying to sell high end stuff. Advertizing with "cheap" may be the converting keyword for your competitor, but not for you.
What you can do is extensive keyword research. Try to find a great number of keywords based on your knowledge from you business. use ubersuggest and keyword planner ( do not kick keywords out, you want many!)
Then run all these keywords against google organic and paid. Check for what keywords your competitors are paying. (us a tool like Advanced Webranking)
Always keep thinking for yourself! Some competitors might not have their campaign in order so they spend money on everything.
You could also put the keywords in the keyword planner and see how much is being paid suggested by the keyword planner.
But all this is still guestimation. The keyword planner is not that good and reliable. you will have to test for yourself using adwords. if your adwords budget is limited, limit it to a specific area and extrapolate the results to the rest of your countries.