As for "problem free" - the issue with V2 boxes is that shells break from excessively upgraded springs in non-reinforced shells. Regular shells can break after time under a light spring too, but it's less likely. They don't have any more internal issues than any other version gearbox. A moderate velocity (under 375) in a reinforced shell will last a hell of a long time problem free. V2 shells usually break when people try to push them too hard or abuse their guns with a lot of dry-firing.
As far as I know, the only MP5 that comes with a reinforced shell out of the box is a Classic Army. As far as I'm concerned, they also make the finest quality metal bodies of any airsoft MP5. They tend to cost a bit more than the other brands, but will last a long time.