Just a couple words of advise from someone who was in your same boat about 1.5 years ago...
I was dead set on getting a diesel (as in, that was all I looked at and drove) but ended up getting a vortec gas. Make sure to do your research on the trucks you're looking at. No matter what year diesel you are looking at, they all have their problems or things that need to be done to them. For example, the 01-04 duramax diesels (LB7 series) have their injector problems, which is an expensive repair when it comes time to replace the injectors. And the 6.5 diesels need their PMD module relocated and plastic cover taken off the top of the engine. So just make sure you know what problems may arise on the truck you are looking at.
The 6.5 and the duramax are not even in the same class. I owned a 6.5 for 4 years and after driving duramax trucks, I can honestly say that the power is no where near the duramax engines. I think my 6.5 (was a 1998) had a towing capacity of only 8000 or 9000lbs, which is even less than what my current 6.0 Vortec can tow. The 6.5, while a decent engine for the most part, is slow and does lack power compared to other diesels. So if you are looking for a diesel for power/towing, go with a duramax...you will thank yourself when towing heavy loads.
Any particular reason the truck must have 150K on it? I remember when I was looking, I was hoping for something around that same mileage but I could only find trucks with higher miles (over 250k) or low miles (under 75k). The low mileage trucks were over my budget at the time and the higher mileage trucks just had too many miles on them for what I wanted.
From my experience I would look at duramax trucks in the '04.5 - '07 years.
When a 6.5L is idleing, if you get white it could be either injectors, injection pump, glow plugs. Black is turbo. The 6.5L will blow black smoke if you'd stomp on the pedal, under load, or if tuned to do so but its not going to end well. You want to avoid that rolling coal black smoke, that is notoris with a cummins. GM turbo's do not like the heat build up from the Exhaust Gas Temps, they hit the danger zone (if I can remember) at 1100. Keep running that turbo hot and you'll find yourself replacing a $800+ part out of supidity.
I'm not trying to knock a 6.5, I own one and I love the high mpg I get with it being a 1 ton. But they can be finicy, wider aftermarket exhaust is almost a must, PMD/FSD relocation is a must, EGT gauge is a must, and keep a eye on that Harmonic Balancer.