The XK5 is a serious upgrade form the C2D. The leslie sim on the XK5 is good, but not great (but better than the C2D); everything else is excellent. The keybed is sweet. You could start with the XK5 and down the road add the lower manual if you feel the need for a lower manual. I have tried one an through a real leslie (122A) it sounded as close to my B3/122 of anything that I have heard.
The other model you might want to explore is the UHL X3-1. New concept organ, it doesn't use software, it is a hardware solution using FPGA ( Field Programmable Gate Arrays) technology. The latency is very very low and the sound is spot on.
Listen to this, it is the HX3 sound module running through a Jupiter keyboard. The HX3 sound module is the guts of the UHL keyboard: