"minimum requirement" when selecting a pic to work with this addin like min size memory, min osc speed, etc?
the code takes < 6k with no images stored , so the chip needs at least that much code space + what your code requires , ram requirements are modest (but uncounted)
at this point the chip must have at least 1 mssp (spi) module
regarding osc :--- faster = better performance


your 18F2520 should be fine