If you don’t have the cash for a professional synth then the portasound series is your best bet, while it isn’t as versatile as the 680 the 560 is still a great starting point.
While it lacks the 680s midi ports it’s still an excellent performer if you’re on a budget, these “toy” synths are easy to service as the service information is easy to find, they sound great and are great as a starter keyboard synth.