Google Buell XB motor differences / changes / model years
*Site
03-05 similar except some bolts & belt system upgraded in 04, don't ask how ( search it) just know it's upgraded
06/07 brass oil pump gear, oil pump internal, trans , swing arm, case mount for oil line, air box inlet. Closed hole in frame, more fuel capacity
08+ engine revamped with larger/lighter crank increased rev limit on XB12, oil lines, added channels to ECM for electronic ignition over point ignition.
Probably forgot some but that's what I remember short and simple.
As for what you should get, whatever's near you and hasn't been abused. All pretty similar and all motors can fail.
If your ready to buy I'm sure there's people selling.