For hardware, I would go with sata ssds for storage. I also would recommend a separate boot drive as it makes re installing without loosing data a breeze. You could go with a new processor from intel or amd, and make sure it has onboard graphics. You should also go into the BIOS, enable xmp (always do that. It's free performance), and look for power saving settings. You could go for arm, but most arm computers lack expansion options and that can mean you use USB for drives which is unreliable. I think dc atx power supplies are a thing, but I know nothing about them. I would recommend doing ext4 and a raid, ideally 6 or 10. That will give you stability (from filesystem issues) and good reliability. I know RAID is not a backup, but in this case it may be what you want. RAID is for uptime, and it would be able to usually buy you enough time to finish your trip, or you could even have spare drives onboard and fix it while you're out in like 10 minutes. Also, I would highly recommend researching how to power it well. Starting an engine can mess with the power and could fry a computer. I don't know if a dc ups is a thing, but if it is, get it.
For software, use whatever you want, mostly. I always use debian for servers, but proxmox would probably be fine. You can use powertop to make sure it runs efficiently.