You could get the ppd from the Windows Xp driver, or the Generic PPD.
As for running LPR commands, All commands are in the PDL reference guide.
Tray mapping can be done ala CWIS > Properties > Services > Printing > Page Description Languages > PCL > Tray Mapping (Not Color, but it will work)
Usually the best bet is to go with the Windows XP driver, extract the PPD, modify it manually to eliminate the options you don't have on your printer and install via CUPS. In this method you won't have the ability to access Xerox Standard Accounting, but everything else will work. Obviously this is not supported by Xerox, and is not a good method if you don't understand the code in the PPD.