helislave, yes, sorry to be the messenger of bad news but that's it under JAR's I'm afraid.
There are a number of entry requirements for the instructor's course, but yes basically you need 300 helicopter hours before commencing the course, which itself is at least 30 hours.
There have been times in the past when North Sea operators have hired people with a basic CPL(H) and no more hours than the minimum, and no instrument rating. But with the present economic climate, it's certainly not happening now.
Bristows have also sponsored people through their training. Highly competitive, usually an upper age limit and not always advertised. Don't know what the state of play is at the moment.
So financially to get those 300 hundred hours is a nightmare. An alternative route is to train in the USA where it's much cheaper and you need about half the hours to be an instructor. Once you've trained, if you're very lucky and good enough you might be taken on by the school you trained at on a visa that allows you to stay and build up time instructing. This way you get the hours. Only drawback is when you get back to Europe, you've still got exams and flying to do to convert to JAA license. However, arguably much cheaper if it all works out as planned. But, if you don't get taken on as an instructor, what then? As you know, there just aren't any guarantees.
In answer to your other question, simply put, not easy to convert other licenses to CAA/JAA. That is probably the biggest hassle. Easier to convert from CAA/JAA to FAA, or AUS or NZ. As for other countries accepting licenses, it can be complicated, especially when you get down to where the aircraft's registered etc. But for example, in Middle East countries and Africa both CAA/JAA and FAA tend to be accepted or easily converted. So generally you're not talking about sitting all the exams again, at most an Air Law exam and IR test if appropriate. It's really just going from anywhere else into the JAA system that tends to be more restrictive.
Ultimately the biggest hurdle of course is do you have a work permit, visa, immigration approval for the country you want to work in?
Hope that answers some questions. Bottom line is, it ain't easy!