That's a difficult question to answer. The lever action in general is a pretty durable design. I think Marlin has improved on their original design now that Ruger has taken over. The improved design, better metallurgy, and more precision parts I think will make the overall design even more durable.
The parts needed to keep a lever action running for 100 years may not be as large as you think. I doubt that your 1895 will be worn out from excessive use. The Ruger/Marlin 1895 is a brand new firearm and parts should be available through Ruger for quite some time. Even after that aftermarket parts should be available.
If it were me I would look at the parts diagram and parts list and start by picking out spare parts such as springs, pins and various screws. Springs stretch or loose their strength over time and it will never hurt to have spares. Pins and screws are often proprietary and cannot be found in a general hardware store so I would definitely try to keep spares. Many times just in the course of cleaning and maintenance it's easy to drop and lose a screw, or bugger up the slot in the head or the threads, so I would definitely try to keep spares.
After that I would concentrate on spare trigger parts, definitely an extractor or two, firing pin, and then if you really want to, a spare bolt assembly. The possibilities are endless. It all depends on how much you want to spend to feel secure that you might cover any possibility.
The main thing that I would try to do, is to properly maintain the firearm by cleaning it and lubricating it and properly storing it so that the possibility of any damage or wear will be greatly minimized so that spare parts aren't even needed.
You can go to any extreme you are comfortable with, with a spare stock, or any other item that you feel comfortable buying, but at that point rather than buying hundreds of dollars of spare parts that you might never use, you would probably be better off with simply buying a second rifle, coating it with cosmoline and storing it away in vacuum sealed bag and not using it.
