I'm thinking of picking up one of these as a work truck. I was wondering how many miles on one of these I sould start worrying about reliability.

one if you are pulling a load. Remember these are light duty diesel engines.

I've got 450,000 kms on my '96 6.5L. It spent the last 2 years pulling a 10,000 lb. 5th wheel. It's definately not hte strongest engine, but if you are carefull it should be quite reliable.