Hi,
regarding the hour requirements for converting an FAA to JAA IR, if you have anything less than 100 hr (PIC) IFR than you basically have to "re-do" your IR, this applies to both fixed-wing and rotor-wing aircraft.
I'm currently in the process of converting FAA to JAA myself (however, fixed-wing) and to my knowledge in Europe IFR is/must be in a multi-engine aircraft - at least regarding commercial ops.
However, a buddy of mine did the conversion a few years ago (he is a heli pilot) and he also mentioned something about flying a twin-engine chopper (something like a Jetranger or so) in order to get his IR converted. After all he refrained from converting his IR because he was short of IFR PIC time and it would have been far to expensive. And as a chopper pilot you (usually) don't need to be instrument reated anyway.
Regarding the conversion of your CPL, I suggest to check out the following link:
http://www.caa.co.uk/docs/175/Section%201%20Subpart%20D%20-%20Amdt%203%20(JAR-FCL%202).pdf
I'd say it's probably the best, prior to start the conversion, to get in touch with the respective Aviation Authority and see what they say. In the UK this would be
Civil Aviation Authority Home Page
Just as a side note, in order to get a commercial flying job in Europe you should/must have EU citizenship, otherwise it's pretty much impossible the get a job.
Greets and good luck
