Sarph, from one LM to another, I'll answer your questions in the order in which you asked them (as best I can).
You should buy the Gondor quest packs in the order in which you will progess through them. That is, Western Gondor (which includes Dol Amroth) --> Central Gondor --> Eastern Gondor --> Old Anorien --> Far Anorien.
There is no Gondor Combo Pack as of right now. All the Gondorian regions are 795 TP each.
In terms of crucialness (is that even a word?), I'd say Western Gondor and Old Anorien are the best since they have Dol Amroth and Minas Tirith, respectively.
Yes, you can get slotted armour from DA dailies. Also, Anorien-level tailors can craft you some slotted armour and Anorien-level jewelers can craft slotted jewelry. I will look around my house and see if I have any spare LM-appropriate essence for you.
The Pelennor Fields instances are part of the Far Anorien quest pack.
Hope that helps.
"For a really healthy development of the arts, you need an educated audience as well as performers." - Eleanor Roosevelt
Just let me know when you get ready to hit lvl 100 and I'll get armor and jewelry made for you. It will be essence gear so make sure you're saving whatever you come across essence wise. Make sure you're working on the epic quests for volume 4 as well as you'll get essences you can slot from completion of quite a few of the later quests, along with Anfalas star lit crystals and symbols for 2nd Age gear until we can get you 1st age stuff made.
I was able to complete my Anorien tailor crafting skills. I can now make tier 2 Anorien light armor. So I no longer need help with that. I also started crafting my tier 2 Anorien Jewelry last night. Again if anyone needs any light armor, woodworking, food of any level just let me know.