The ELC coolant isnt neccesary. Maintenance of the green stuff is. Keeping proper SCA levels in the regular green coolant is all it takes. If the original owner was a diesel mechanic he probably was doing this. There are simple little test strips to check it. Best solution is to add a coolant filter. The SCAs prevent cavitation, not for the waterpumps sake but the cylinder wall. The cylinder pressures in a diesel are so high that the walls flex a very small amount. The rapid pulsation can cause the cavitation which would eventually lead to small pinholes in the cylinder wall.
This isnt a huge issue. I have personally never seen a diesel pickup have a problem from it but I have seen several big trucks have it happen, usually with close to 1,000,000 miles.
FWIW I have a 95 F350 with a 7.3 and it has 345,000 miles. I bought it with 297 and it was a bone stock turd that was used to haul cattle its entire life. You would never know that now. It makes an honest 400hp on the dyno and I still get an average of 15mpg. Ive got a lot of money into it to get there.
I would never even consider a cummins swap into my Ford! I just picked up a core 7.3 engine that I plan to build over the winter to hit the 500hp mark.