As part of my yearly income derives from driving a truck with a 6.5L Detroit Diesel, I would advise you to look elsewhere. Even as much as I despise a 5.9 Cummins, a healthy Cummins would be a far better option.
If it is a stock 6.5L, then there is so much money you will have to dump into it just to make it a reliable engine, and to avoid blowing it up.
Here are some common flaws of a 6.5L:
Poor coolant flow through the engine. (All years.)
Poor coolant flow, crappy water pumps. (Best were 97+)
Injection pumps that love to commit suicide when you lift pump dies.
Lift pumps that kill healthy Injection pumps.
Lift pumps that provide poor fuel flow.
Lift pumps that turn off due to faulty oil pressure sensor/switch.
Stock turbos are very limited, and leave the high end lacking.
Stock exhaust leaves a very restricted flow and high EGT's.
Pricy PMD's that love to die from heat exhaustion.
Pricy PMD's that require a good heat sink due to high temps it creates.
GM for some reason didn't like to include gauges with these engines.
A 454 of the same year generally produces more torque and hp.
A 454 of the same year has power through full rpm range. 6.5 does not.
Other things to consider with a GM 6.5L:
Injectors are pricy.
Glow plugs are pricy.
Injection pump is insane.
Coolant system all the way around is terrible on most every 6.5 application.
They are very easy to overheat, both water temp and EGT's.
They cannot be heavily modded like an international or cummins.
Heavy modification usually leads to a dead 6.5l Detroit.
A stock 6.5L cannot handle much increase in boost.
A stock 6.5L isn't completely gutless, but it's not as good as you'd like.
A stock 6.5L has generally no guts left after 2500rpm.
A 6.5L gets better fuel mileage than a 454 by a large margin.
A 6.5L is acceptable if if you're not needing to utilize the power.
I can't really think of any others.
Most everything on that list will have to be addressed on every stock 6.5. I have a fairly stock 6.5 with 200k miles with lots of towing 7-14k lbs. If you're using it with an automatic transmission, and you're pushing it, it will downshift expecting to get more power, though that usually is not the case, and you'll lose power to the ground. If you can find the same bus in a 454 or 350, you will have a much more reliable engine. If you want a diesel, find a E-bus with an international.
A am loyal to GM, but the 6.5L is a very disappointing engine. I wish I could swap in a bigger Detroit, such as the 466. My next truck will likely be a 454. If I get a bus, it will either be a big block gasser, or a DT466, or maybe a 444 if the bus is perfect for me.
Not trying to rain on anybody's parade, but I am a 6.5L Detroit owner, and I would not recommend it.