The Yamaha PSR2100 has all the reverbs, echos and other DSPs one would ever use. I also happen to think that the harmonizer sounds more than adequate for live use.
I do use an external mixer some of the time. It is usually Mackie 1202 VLZ, but I also have an 8-channel Behringer that seems to be just as good (except it has a wall wart power supply).
You would need the Nanoverb.
DonM