Someone had major problems like blown headgasket (which in a 6.5 means high probability of cracked cylinder walls and or cracked heads) and sold it. Then the new owners: they rebuilt part of the engine, couldn’t get it running right, and undoubtedly threw more extra parts at it, and sold it.
There is no way I would try adding any high quality expensive parts on it if it were “see if it runs scenario”.
Rip the engine out, and see exactly what you have.
If they couldn’t get it, why think anything they did is right?
You’re 100% they knew to not reuse the headbolts?, and how to measure timing chain slack?
My preference: pull it.
Extra work, yes. New set of head gaskets and headbolts or studs, yes, 1 tube of permetex, yes. And IF everything else is correct then inside the engine, it goes back together and you know you will have a good long life from it. However, anything they did wrong that you find instantly makes it worth it.
If you dont pull it apart: compression test, leakdown test, coolant system pressure test. But i still would run it with existing turbo and everything else for several hundred miles before adding parts like ATT that if the engine lets go because they put it together wrong, and destroying a good turbo.
Just my 2c