Better plan: pick something that is currently and widely used in aerospace and military applications and the world will preserve working systems for you.
I have personal experience with this. 15 years ago, just before I left an employer I had worked for for some time, I took a number of Digital Alpha workstations off their hands; they just gave them away after about five years of use and replaced them with newer workstations.
It turns out there is a thriving market for this hardware because aerospace and military outfits used it for their work and today they still have drawings and material they need to deal with in original form. They have migrated the original material to newer systems, but they also still maintain the equipment and software needed to get at the material in its original form.
They pay through the nose to get replacement parts and complete systems in working condition, so a salvage market has emerged and people prowl around trying to find caches of ancient workstations. Doubtless this will be ongoing for at least another ten years, and the prices will escalate accordingly.
So if you need to ensure there will be spare parts and systems at your disposal a quarter century from now, find out what Lockheed and Boeing are designing today's jets with and use that stuff. It's built well and people pay dearly for it when new, so it tends to be carefully preserved; it's hard to trash something that cost $20k, even if it is wildly obsolete.