The 6.5L is very much like the 6.2L. It is a light duty diesel. It is not overly powerful, but it gets great mileage and does not slow down a whole lot when loaded down.
Yes, they can start to run warm but any engine will when you overload the truck (which most people do when towing). Heck, I managed to overheat my old 07 LBZ Duramax just towing my Tahoe on a 20ft gooseneck.
There are a few things to consider when owing a 6.5L diesel truck:
1. You MUST run fuel additive to keep your injection pump alive. The only additive I would recommend is the Standyne stuff. It adds essential lubricity to the diesel to prevent excessive wear caused by the ultra slow sulphur diesel (ULSD) sold at the pumps. The sulphur was the agent that added the lubricity to the diesel.
2. You need to keep a close eye on the harmonic balancer. Lots of folks complain that the 6.5L engines break cranks a lot. This is almost always attributed to a failed balancer. If your balancer is missing chunks of the rubber ring between the two halves, is cracked, swollen etc it needs to be replaced immediately.
3. The injectors are only good for about 100k with ULSD before the nozzles begin to fail.
4. The optical sensor in the injection pump is quite sensitive to colored fuel. The engine will drop into limp mode if it looses count on the high resolution wheel due to the optical sensor being unable to see the tone wheel due to the diesel being colored.
5. You need to make sure that the vacuum pump is up to snuff. The wastegate on the turbo needs 20inHG of vacuum to seal shut. And less and you'll be wasting boost. My old 6.5L in the Tahoe had this problem. Every once in a while the vacuum pump would drop out and I'd lose all my boost. The black smoke was pretty impressive to look at but I sure lost a lot of power haha
6. The PMD is a common failure item but not the only failure item. Don't automatically assume it is bad if the engine is misbehaving. Do some troubleshooting before you drop the cash on a new one. Heat is what kills these things. If you want it to stay alive then install a relocation kit with a big heat sink.
If you are going to buy a 6.5L diesel truck, make sure you check the glow plugs. The engine is totally depenant on them working properly to cold start. They are not super expensive (12 bucks each for a Delco 60G) and make a world of difference for starting. The other key to a successful start is starting RPM. If your batteries are getting weak, or if the starter is becoming worn out then you should replace them to make sure the engine turns over as fast as it can.
Yes, these engines are notorious for cracking in the mains and between the valves on the heads, but it is not that often that it actually causes a failure. I have never seen or heard of anyone suffering a bottom end failure due to cracked mains. Nor have I seen many people suffer problems with cracked heads. I would not worry about either until it becomes a problem.
The 6.5L is a simple engine. It is reliable if you take care of it. It will get you great mileage and will also pull quite decently well as long as you don't expect it to pull like a modern diesel. Give yourself lots of time to get up to speed and it'll get you there. The Chevy trucks are comfortable and most are loaded with lots of creature comforts. I like the Dodge engines, but the trucks they are wrapped in are inferior to the Chevy stuff. Fords I don't have a lot of experience with, but they seem to work pretty decently as well.
And yes, my Tahoe hauled Horton with the 6.5L / 4l80e a couple hundred miles at 75mph without any trouble

I was leaving a first gen Cummins behind in my black smoke (when the turbo dropped out) on some of the hills. But I also have 5.13 gears. That helps some haha