I have a 01 and 06 ISX  engines now . We ran a 04 ISX to 1.1million over the road  put  lots of turbo and emmission related items on it not cheap. If you want a true non emmission engine  2002 is about as new as you can get. The era of the 6nz  C-15 < we had one  a true horse and dependable. The 01 ISX has been great also a simple turbocharger and no egr . The last year for the N-14 in  a Kw was 01/02. I had 1993 KW with a N-14 and was happy with it. I suspect parts are cheaper for a N-14 also .  A 12.7 Detroit of that era was a great engine also. We bought the 01 KW new and all the other trucks ,5 of them with a around 400,000-500,000 miles . Seems to be kind of a sweet spot for reliability versous cost, Just  2 farm trucks now.