It is easier to speak with someone else's money but here is what I recommend while you are "In the neighborhood"
Obviously you need to find out what caused the overheating that led to the blown head gasket in the first place.. Be it bad water pump, or radiator etc...
Of course both head gaskets and bolts. Have a good professional do this type of work and also check real close for head warpage/craking, block damage, clean/decarbon and inspect the top of the block area for piston damage...
Check valves for damage, clean/hot tank both heads, inspect for other damage.
With 100K on the clock, I would recommend r/r the timing chains and associated hardware.
Water pump if it hasn't been changed at all.
good cleaning of lower block area at least as much as you can get access to
Valve Cover Gaskets.
If you can afford it maybe some upgraded cams, I am sure they are out there for 5.9's
Clean and inspect Intake and exhaust manifolds for damage and at the very least get a good cleaning.
02 sensors, not very expensive, and you probably won't get much easier to access to them in the future.
R/R drive belt with Good Year GatorBack Serpentine drive belt.
I know it sounds expensive but you will basically have a new engine and many more (hopefully trouble free) miles.
Once it gets back together full cooling system back flush, radiator cleaning if not a full replacement.
IndyD will probably chime in soon, but I would venture to guess his recommendations are going to be Prestone 50/50 Green coolant and getting all air out of system...I am not sure if the 5.9 has the bleed screw like the 4.7 but if so, it is there to get air out of the pockets in the coolant passages.
Again these are my reommendations, again it is easy to speak for someone elses money but these are good quality fixes and should hopefully give you a total overhaul and keep the D going for quite a while.
At a very minimum ask yourself these questions......How much extra time and money is it going to cost if another internal engine component fails after the block and heads are put back together?
Don't forget in progress photos!!!!!!