Any refurbished/used x86 is almost always a better choice than a the newer RPi's. By the time you get done bringing the RPi up to the spec you need, it's almost always more expensive and less reliable than something x86.

If you fit the envelope, the Beaglebone Black has been out forever. It's not fast. It's doesn't have super modern interfaces (Displayport, PCI-E). It's not super tiny.

However, it is solid. It actually runs in the 500mA USB envelope and doesn't need a heat sink. It has eMMC so you don't have to fiddle with garbage uSD cards. It is incredibly well documented thanks to TI. It has a useful number of I/O pins (unlike the measly amount on the RPi). It has tons of the kind of basic hardware interfaces that you need to interface to things. The real time processors on it can often substitute for FPGAs. There are industrial versions for $10 more than the standard $50. And the software follows bog-standard mainline Debian rather than being some weird, undocumented, bodged-up thing that needs to boot from the GPU.