first off welcome to the cummins family... second the difference between the 12 valves and 24 valves is that the 12 valves are all mechanical from mechanical lift pumps to mechanical injection pumps so on and so forth..... the 24 valves use electric lift pumps and stuff like that.... and finally things to loo for in a 98.5-02 cummins is if you buy one buy a fuel pressure guage asap and do not let your fuel psi to get under 5 psi or you'll most likely be replacing your injection pump shortly which isnt cheap..... anywyas i'm sure more people will tell ya what else to look out for also