. . . (a) Do you want to develop sounds yourself,
. . . (b) or would you be content with choosing from (say) 1,000 sounds included in the manufacturer's sound library ?
If (b), a kind of synth called a "ROMpler" (which essentially plays back samples) would do the job. It's easy to find those with high polyphony counts -- easily high enough for Bach, with a pedal in use.
An example of such would be a Yamaha MX88 (there are less-expensive versions with fewer keys -- but the MX 88 has a fully-weighted keyboard).(*)
If (a), you need a fully-programmable synth. For _any_ such synth (unless it's computer-based), there will be a polyphony limit -- essentially, how many pitches can be sounding at once.
Since you have only ten fingers, you might expect that a polyphony of 10 would be adequate for anything. But many sounds keep going after their key is released, and most synths offer a "sustain" pedal (that works like a piano's "damper" pedal). So it's easy to play something that needs 20 or so simultaneous pitches.
The Studiologic Sledge 2.0 (mentioned above) is a reasonable choice, in that realm.
. . Does yamaha produce some machine with the same synthesis capabilities as modx but in a non keyboard format?
Without a keyboard, how are you going to "play" Bach ? You don't mention that you already own a MIDI-capable keyboard.
(*) -- if the MX-series _is_ fully-programmable, I apologize for the error. I don't think it is.