Welcome.
That is a pretty open request. What you are doing with it, amount you have to spend, etc all comes into play.
What mechanical knowledge do you have could help.
Do you want to buy cheap and do a bunch of repairs yourself or buy one really well maintained for a lot more money?
Oil pressure, blow by, compression, and various upgrades for the engine are up for considering.
Welcome to The Truck Stop. There's probably more 6.5 knowledge here than you can imagine. Look around and spend lots of time reading. Everyone here is helpful.
First thing to do is get a book. I have a Chiltons and a Haynes book for these trucks. The haines gives no information whatsoever, get the Chiltons, it will cover most of the general repairs that these engines will require.
And, during the mean time, look into the sticky portions of this thread.