Depending on how big you want to go, you might want to consider that, rather than using several general-purpose RPis (e.g., the RPi 3B+, which is their latest and greatest), you could use one of those as a primary unit - the one used for the user interface and other peripherals - and then use RPi ZeroWs or even RPI 3 CMs (if you have the electronics skill to work with industrial SO-DIMM sockets) for the rest, with the primary acting as both user interface and as a manager for the others.
Conversely, and in conjunction with that, you might consider other kinds of SBCs for subordinate processors; there are probably some aspects of this that could be handled by a less powerful
Arduino, while some highly parallel operations you might want some kind of
DSP, or something like a
Parallax Propeller or an
Adapteva Parallela. You may even want to look at an
FPGA or two, though at that point ambition is probably getting out of hand.
And I would be remiss (and risk
raising Zaval's ire) if I didn't suggest considering some of the less well known SBCs, such as the
Libre Renegade, which unlike the RPi has no proprietary components to speak of (that is, you can write all your own drivers, which you can't with the RPi because those use Broadcom wifi subsystems that only work properly with the provided binary blob drivers).