Your question is about like asking how high is uP? Or “what do I have to do to live to be 100”? I have not had a NH310 baler, but I believe the feeder mechanism is covered by the sheet metal of the baler like a 273. That should protect it from a lot of elements. In photos of that baler the knotters appear to be under a metal cover and if no rain has blown in and rusted them, they should be OK. The real challenge will be the bale chamber. If it was left full of hay and has been rained on a lot, the bale chamber will be heavily rusted and the plunger might be frozen in place by rust. I would start by making sure that the baler will rotate and the plunger is free. Empty the bale chamber and try turning it over by hand.
Remember there are a lot of ugly old rusty balers on the outside that are beautiful, strong and healthy on the inside.