Hi
Most all keyboards have lots of ballad styles/rhythms,
and also a lot of additional sofware to fit most
all kinds of taste.
The only person who can answer your question
properly, is most likely yourself.
Only to visit the dealers or other places where
those keyboards can be played on, and try this
out by yourself will give you the answer about
what keyboard who suit your needs.
Then you have to think about how you're going to
use the keyboard, by use only the "onboard" or
additional styles, or do you want to make those
by yourself.
If you like to do this kind of things easily,
then you should try out several brands to see
how they work and sounds.
Kind regards!
GJ