welcome to the forum!
1. great question! you're right, that's not talked about very often on forums. might check 'bobistheoilguy' to see if there is something worthwhile there. i'd like to hear more about what spec filter would be worth it, but for sure, you should only need motorcraft oil (and fuel too lol) filters and a quality full synth oil while you are DD'ing it.
2. just curious, what kind of coolant does/has the truck had in it? if you went with a good ELC (EC1 rated coolant) like rotella's read stuff AND a coolant filter, i would feel much better about running the truck with the stock water pump and radiator. Rather, just run it til it needs replacement.
3. You might find some good answers in the 'Transmisison Related' subforum on here. For sure though, a good heated flush or a series of flush-n-fills is highly recommended in the short term. it's hard to get much over half the fluid out by draining it just once.
4. An Edge monitor would be great! you can install a pyrometer to monitor egts
while towing that 5th wheel. otherwise, i love my scanguageII. and learning to love my elc obd2 bluetooth adapter (work with your smartphone & is only about $20). if money isn't an issue, that edge with pyro is hard to beat for monitoring IMO
. ever think about installing a custom tune? might consider going with a full blown programmer so you can load a tow tune when you want it.
5. This is the biggest question lol, i'll start with ... blue spring mod, new oil cooler, metal CAC
tube on driver side, strait pipe or CAT
delete on exhaust, zoodad mod, maybe some injectors for towing when the time comes (with a nice tow tune), HPO updates (standpipes, dummy plugs, STC fitting) ... list goes on my friend! fumoto valves on either side of the block (ease of draining coolant), 6.4 starter, upgraded water separator drain cap thingy, CCV
reroute, load supports on rear axle (air bags, AALs, timbrens, supersprings, plenty of choices)... i believe theres an updated turbo drain tube as well... you could go hog wild and install ceramic coated uppipes lol