Where to find a DDEC 4

Recently bought a tri-axle dump truck (99 freightliner) with a blown 11.1 ddec4. My plan was to find a 12.7 to replace it as the 11.1 leaves a lot to be desired and I know the 97-99 ddec4 is like the holy grail but as a local driver that doesn't travel more than 100 miles from home, I am not worried about the ELD. If my research is correct I should be able to run a 99-2003 12.7. Does that sound right? And where does a guy find a good running one? Was hoping to pull the 11.1, swing in the 12.7 and go to work. If I can't find a ddec4 what would it hurt to run a ddec3 and maybe even later on upgrade to a ddec4 ecm or would the newer ecm even be necessary? Would the ddec3 button up where a ddec4 came out and everything work just as the ddec4 once did, that is if I can't find a ddec4. TIA

 

I have found a ddec3 but not sure it will work. What am I losing or disadvantage(s) am I looking at by going from the ddec4 11.1 to ddec3 12.7? Anything specific I need to look for in a ddec3?