Instead of floating the scope by going battery powered, you can use two scope channels and math to create a differential probe. Or buy an actual differential probe, of which Micsig is a popular option.
Are you wanting "handheld" or "portable"?
Do you actually need to decode CAN bus, or just have a look at the signals to check for faults? A lot of CAN bus troubleshooting can be done without decoding.
For the money, you do get a lot with the new Rigol & Siglent 800 series scopes, especially when you consider hacking. But what if your price target is $200?
eBay is still a great place to find deals, but you just can't treat it like a regular store. Set up search alerts and wait a few weeks to see if something turns up.
With regard to "future proof", it's for sure a loaded term. A wrench might be "future proof" because it's always going to be useful by doing the same job forever. So in that sense, even test equipment could be "future proof" depending on various factors. There is a reason why some used test gear can still demand a premium.
Products that need software and/or drivers to function should get extra scrutiny before purchasing, however. In my opinion, Ethernet and serial are good physical interfaces, GPIB is acceptable, USB-only could be problematic. Also look for LXI and SCPI support. Also, even if the device has Ethernet but still requires proprietary software, you might get pretty far with virtualization (if you still have the software), but a proper open standard is better.