The eNMEy uses Impulse/Ion barrels.
A longer barrel isn't going to improve accuracy or range, but it will make the eNMEy quieter, and depending on your style may make the gun easier to aim.
14" is the most popular aftermarket barrel length.
As for the manufacturer, there are plenty of good options out there, but I certainly wouldn't pay over $40.
and Custom Products
are both good places to start.
I wouldn't bother with a barrel kit like a Freak, either; for a simple mechanical gun like the eNMEy it's total overkill and to be honest I don't think non-pump users are going to get much out of underboring anyway.