As far as running costs, our 04.5 duramax has 120K miles. Haven't had any warranty work. Haven't had to do anything other than prescribed ongoing maintence, fluids, filters etc.
Only thing I did do that was not specified was to have the local allison shop change out the dextron transmission fluid for the allison transynd fluid and installing the standard deep transmission pan. (GM spec'd dextron and the short pan for the allison that comes std.)
Regular maintence costs of a diesel in my experience is about the same as a gasser. yes it takes more oil, yes fuel filter needs to be changed more often, but in the grand scheme of total costs, any difference is down in the noise.
Neighbor has a 01 ford 7.3, over 250K miles and is starting to show it's age, but again, darn reliable. Only issue was the original transmission that was replaced under warranty. Other than that pretty much regular maintenance and now some stuff needing replacement, but not unexpected with the miles on it IMHO.
Find a well maintained used truck in good condition, gas or diesel and you'll likely be happy.