Have you checked CASIO WK3200 or WK3700 ? It can do those splits and assigned any voice to the split sections + also another section for layering in the upper section.

It has 76 keys (the DGX 500 has 88 keys) but should be plenty for most people and the CASIO received rave reviews even by the pros here. But I think it sounds decent when it is amplified thru a sound system or even a home stereo. I think the built in speakers does not make the sound as good as it should be.

Also, try to buy from store that allows 30 days trial period with full refund like many stores here in USA.