ATR is not a common plane, true, but then there are not so many companies flying them. And YES, it is VERY difficult to find a job even with 500 hours on the ATR. Just send fake CVS and see how many interviews you get. Asian companies are not interested in FOs as they have their own local pilots and would rather pay them a TR, so the only options you have are the few caribbean ATR ops or in the Pacific, but good luck with that.
And yes, Farnair will probably thank you for working with them after the 500H, which might take up to 18 months ( 300-350 Hr/year ).
So please think carefully before accepting their offer. Look at what LionAIr and RAM have done to the other 737 pilots...