Frankie,
You can use ANY keyboard amd midi it INTO the SD1 for the tone. Buy whatever feels good and fits on your stand, and write a patch for sequence play that allows for the bass to be triggered from an external source. It can even be the right hand sound, if you write a patch that has a split built in already.

As an alternative - check out the Roland/Edirol SK880pro - it's 49 keys, 64 voices, and feels great. Should be right in that price range too, I think.