thinking of buying

Well, I would go for it. The engine should be covered for a little while longer and injectors are covered for 7yr/200,000 miles. Are you that upside down in the truck that you own now? There are many dmax's that I have read about that have 400,000 + miles on it. As long as you keep up with the normal maintenance (fuel filters, oil changes, transmission filter and fluid, and diff fluids) you should be ok. There is other stuff that wears out, but I would call it normal wear and tear and not an issue. The only longevity problems that you could have is from thowing a big program on the truck. If you stick to a predator or other flash scanner like it, you should be ok.

The block of the duramax is cast iron too...it just has aluminum heads. Check it out here: The Diesel Page News

That link tells about the 2001, but it is the same as the one in the 2003.

I would look at not taking out a loan for so long if you can help it though. It would make you upside down in that truck if you ever went to get rid of it....not to mention all the extra interest that you pay.