You may also want to consider a Hydrasynth Explorer. 37 keys with poly AT, and a very nice subtractive synth engine with a ton of fun bells and whistles. Also a chord mode and arpeggiator mode that may be more powerful than the Keystep 37.
I’m using the larger Hydrasynth Keyboard to explore MPE. The core synth engine is powerful enough to make very interesting sounds on its own, in drone mode or with a more ordinary keyboard. Adding in PolyAT and a giant ribbon controller combines very well with such a nuanced synth engine.
If the current setup is computer, Keystep and Digitakt, then the small Hydrasynth Explorer could be a good replacement for the Keystep.