My family has ran a 5400 chopper,619 engine, with 3 row head for 6 years with minimal troubles chopping 60-80 acres a year of silage and then 10-15 acres on earlage.  On the chopper head I would take a look at the gathering belts, gears and shafts on the head to ensure they are not rounded off that was the biggest trouble we had with the head.  We were going to go up to a 4 row head but JD advised against it saying the gear box was not heavy enough and many guys had to replace gear boxes prematurely.

Before it hits the field this fall we are going to rebuild the hydro stat as it is getting weak so you may want to have a pressure test done.  Check the augers on the pan below they may be worn, if so poly wearshoes are a good option that works nicely to repair the augers.  Engine wise no trouble plenty of power.  AC is the only thing that doesn't work on ours otherwise it has been a good unit.