I've always been a Yamaha man, so I may be a bit biased here, but consider looking for a used Yamaha EX5r module. They can be had for about $500. The piano sound is decent. With a little tweaking, it's pretty damn good.
However, the electric piano sounds are fantastic. The Wurli can be clean or dirty and the tremelo is right on. The factory Rhodes sound leaves much to be desired, but the ex5 user group (www.ex5tech.com) has a bunch of free sounds, one of which is called the SkiRhodes (Ski is the guy who made it) which is really really good.
I have the Yamaha EX7 (which is the 61-key version of the module) and I use it with my r&b band for piano, electric piano, clavinet (it has great clavinet sounds, too), horn section sounds, strings, etc. I bought it off eBay back in 2001 for $400. You could look for one of those, too but the downside is that they are heavy. The rack module is much lighter and smaller and actually a bit more powerful in the synth department.
And finally, yes you should be able to use your XK-2 as a controller for the rack module. That's why a module would be your best bet, because you already have a keyboard to play it with.