You should consider the 6.7 cummins head gasket if you are planning to replace your head gasket very soon. Generally genuine Cummins gaskets are what you would want to purchase if you want to get good value for money.
It is important to keep in mind that milling the block while it is still in the truck is expensive and risky. It is not worth the risk. In most cases, the block does not need to be milled, but it is important to have the head machined regardless of what shape you think it is in.
If you have to deck the block, it is better if you just buy a new engine. The 6.7 cummins head gaskets do not have much leeway when it comes to clearance between the valves and piston heads. So, it can be extremely dangerous to take any amount of material off the block deck. While it can be done, you are better off buying a new engine. Chances are you will be spending roughly the same amount of money to have all that work done as you would spend on a known good engine out of a wrecked truck.
It is imperative that you get the right thickness head gasket. Make sure that you check the block deck with a straight edge and a flash light, and get head studs.
All original 6.7 cummins have enough deck height to get one resurfacing and keep piston protrusion in spec. So, it may not be necessary to get a new engine just because you need a head gasket. Max protrusion is about .022″. This is why the top of the oil cooler has a little step up to the top of the deck. In case you have no step up, then you know the block has already been decked once.