Just so things are clear. A Keystep has either 32 or 37 keys, depending on the model. The ROLI Rise 2 has 49 keys and cost a lot more than a few hundred dollars. Perhaps you are talking about the recently announced ROLI Blocks keyboard ? It indeed has 24 keys and will be a few hundred dollars. It’s an upgraded re-release of the old ROLI Blocks. See my post on it.
Unless you are talking about a used ROLI, which i’d advise against.
So i assume you mean the new ROLI Blocks keyboard.
I also recall you posting about something like this a while back.
So you have Ableton Live which allows you to control it with MPE, and could be controlled expressively with an MPE controller.
If you have questions about HW or SW synths that are controllable by MPE, that is a separate question.
So I’d say it is reasonable to consider the new ROLI Blocks keyboard.
The suggestion above to compare this with the Osmose, while the Osmose is wonderful, seems way off from the direction you are heading with your question, considering that that keyboard synth is so much more expensive than a few hundred dollars, and isn’t setup as an Ableton Live controller.
I want you to take a look at the KORG Keystage keyboard controller over in this thread.. The 49 key version of it is more in line with your cost guidelines. just slightly above in cost, has more keys than your Keystep, is set up as a real controller with all those knobs, is set up to talk to Ableton Live, has the MPE and a poly-aftertouch keybed, AND as an added wrinkle is set up for MIDI 2.0, to help it stay current with future developments in that direction.
ADDED : Oh i forgot, the Keystage has this really powerful chord and chord arpegiation feature built in.
It could also be used as a plain MIDI controller to your Elekrron devices.
It may not be a fit but take a look.
What do you think SouvlakiPlaystation ?