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"That means that a Duex 5 or similar is needed." not true. Klipper is running on Raspberry Pi and may control multiple 8 bit boards. So my setup for 4 Z motors is: Rapsberry Pi +2 Ramps 1.4 boards what gives access to 10 stepper drivers without any expansion boards. Klipper itself is 32 bit application so all calculations of motor movements are 32 bit. It sends simple 8bit commands to MCUs (Ramps 1.4 or similar) to drive stepper drivers. So no need for 32 bit boards, even so, it can work with 32 bit boards. ABL is already implemented in Klipper and also dual or quad Z endstop configurations. So you will get this out of the box. If 10 steppers are not enough, connect one more Ramps 1.4 and you will have 15 steppers. Also instead of running Klipper on Raspberry Pi, you may run it on any computer/laptop with compatible Linux and USB ports.