Really depends on what you want to do with it. Towing, Racing, Sled Puller etc. There are so many things you can do. I can't speak for the Duramax's but I would suggest the following depending on how much you want to spend:
Aftermarket Intake
Aftermarket Exhaust
Gauges
Just depends on what you want to do really, but that is a start.
Then it's things like, Fuel Boxes, Transmission Upgrades, Propane, Nos, Injectors, Turbo. uhh I'll stop there.
But once you start, you get the disease; I think the medical term is "Dieselitis" if I'm not mistaken.
I got bit and now I'm playing and paying.
Tom
Edited by: TomHill