I don't know if there is a SPI driver like the RPi from Linuxcnc (this is a dedicated driver for only the Rpi3/34), as per the RPI the /dev/spidev driver probably wont give you the speed you are after.
Unless you already have a 7c80 or 7c81 I'd tend to stay away from the SBC's and go for a small form factor x86 system (if space is a concern) and an ethernet mesa card. That way you wont be stuck when you need to update\replace the computer. Ethernet is everywhere, SPI seems to be the realm of the non-x86 platforms, and each SBC will require it's own driver, which requires someone to develop & maintain it and you are at the whim of the manufacturers\dealers regarding availability........and the infinite number of models.
Another thing to take into account some of these SBC's require messing about with the uboot loader, or whatever flavour of boot loader is used, to use a realtime kernel.
They have their place, it just depends how much time you are willing to spend and whether it will be obsolete in a couple of months to a year.
