How do you feed silage now? If you have a vertical mixer already (that can handle the silage bales) making silage bales may be a very decent option for you.
If chopping is the way to go for sure...
Loading trucks can be done, likely requires a vertical chute extension, we had two separate with ours when we sold it.
I am assuming you are referring to headers for the chopper? Never ran a corn header on our pull type, just a pickup header that we used for barley and alfalfa mostly, sometimes oats or triticale or rye. Often found that a big swath (30+ft) would power things out too much (we ran a JD 3975 pulled by a JD 4640) could have used either a better transmission (power shift would've been great) or more overall power (if the driveline would handle it). Frustrating to try to load trucks on the fly if you are starting and stopping a lot
Regular wear items will be pick up teeth, knives, blower paddles, etc. Can get decent quality from aftermarket like kooima, etc. Higher hours will start seeing wear on cross auger, blower chute. Some bearings here and there. We did buy a complete new chute from kooima for our self propelled last year, and will be paying more attention to the replaceable liners before we wear through the entire body on this one.
All in all, you can absolutely make decent silage if you have the man power. The one fundamental difference between JD and NH pull type vs self propelled is the cross auger. They both have it and it is a drag on the power and efficiency. It's where ours would plug up the easiest (where auger dumps into blower). I honestly don't know if just pouring more horsepower to it can fix it or if it's just a bottleneck, no way around it. Setting the cut length shorter does help to keep it from plugging as much, but longer lengths do pack and feed better in many situations, so it's a bit of a trade off. This is one big benefit of the self propelled, everything is one big line, cutter feeds right into blower. There are one or two manufacturers of pull type choppers that don't use cross augers. jF Stoll is designed where the cutter chops up and throws material straight into the chute (no room for a kernel processor though, so not a great choice in corn) and Dion makes one with cutter, kernel processor and blower all in a line. But they are both pretty small companies with limited dealership coverage.
So yeah, that's what I think I remember. We never put a kernel processor in ours so no idea how that ties in, likely crammed in there somewhere. Take most of the shields off and leave them in the shop, you will need to get in there and grease things more than anything. Aftermarket walking tandems are likely a decent option if you can find it. I liked JD's individual knives vs NH's one piece curved units, but either can get the job done
Good luck, feel free to ask any other questions that float through...