If the HPOP is the original one, I would replace it with an Adrenaline HPOP (stock output). Get the Ford gasket kit for it though.
If the dummy plugs and standpipes haven't been updated, then update them.
You have to decide on your own on injectors. Some people get 200k+ and some don't get 180k. If you do get injectors, stick w/ the Ford Remans (from a reputable source) or the new Pure Power injectors.
You need to monitor your coolant and oil temps (after the engine is FULLY warmed up). This will tell you if your oil cooler is plugged up (on the coolant side). The oil should not ever get more than 15 degrees (and I like to see less than 10 degrees) more than the coolant. If your oil cooler is plugged, you will need an extensive flushing of the coolant system and then you should probably change out the oil cooler. Stick w/ the Ford oil cooler from a dealership or from FICMrepair.com. Oh, you will also want the upgraded screen for the oil cooler sump (HPOP screen).
The Late 04 has the weak EGR cooler. If you aren't doing an EGR delete, you should install an EGR cooler from BulletProofDiesel.com.
If the cab was coming off, then it would be a great time to do the cab isolator bushings. They seem to fail around 200k miles.
Consider doing the heater hoses, etc that are hard to get access to without the engine removed.
Glow plug harnesses get brittle w/ heat and age. Probably need to plan on replacing those. Glow plugs are easy to get to w/ the engine out and aren't all that expensive.
The cam and crank sensors aren't REAL prone to failure, but they can leak oil. You can get new seals or replace them altogether.
ONLY get the Ford sensors, gaskets, and seals. They are proven to be reliable! Aftermarket is a big risk in these categories IMO.
The 04's have had a lot of engine harness problems. Yours may be fine or may be getting brittle, etc. Only you can tell that. The harness is expensive, but no better time for it.
Your turbo may be fine or it may need to be cleaned. Probably worth cleaning at least. You can get the turbo rebuild kit from KCturbo. I would get the 360 kit or the 360 extreme kit. They have the unison ring there for a decent price. Might want to get a new turbo oil feed line and oil drain tube also.
Exhaust manifolds didn't come w/ gaskets. No better time than to buy them and install them.
There is an "intake to front cover" o-ring that you probably will need. #W301767
It would be a great time to install an EGT thermocouple in the exhaust manifold piping and if you want a larger exhaust, then now is the time for that as well.
AutoNation White Bear Lake is a great place to get the parts. Shipping is a little high, but prices are great.
The 6.4L push rods are something to consider. Your choice of heads or head machining may play a part in your decision there.
A lot of people are liking the o-ringed Kill-Devil heads. UCF has some good reviews also.
Your model year takes 18mm (locating dowel) heads.
Certainly go w/ ARP head studs, and get them from a reputable source. Counterfeiters are everywhere.
FelPro head gaskets are good.
You might want to call KillDevil Diesel to discuss the heads and head gaskets
https://killdevildiesel.com/
Killdevildiesel sells a good top-end gasket set also.