Just a small contribution: at least in Spain, where I converted my license, if you have your national (ICAO) ATPL you will "only" have to fly 10 hours (5 in single engine and 5 in multiengine). If you go with a CPL you will have to fly more hours, including simulator time.
Therefore, if you can get your local ATPL do it before going for the flying part of the conversion. My advice is to find out in the country you are going to convert your license; I have learned through time that JAA is not the same throughout Europe.