For Mana regen;
W/O AA's cap is 15
W/ OoW AA Expansive Mind cap is 20
W/ TSS AA Labrythine Mind cap is 25
Not sure about the steadfast servent, havent dueled anyone except to ghetto gate.
As far as DoN armor, like mentioned the GM armor is tough to beat outside of raiding, at least for the BP and Greaves. Anything lower then GM armor you can probably get equivalent or better in the bazaar. At the higher levels for GM armor there are stones dropped in DoDh raids that can be used to make some very nice augments, below is an example.
https://everquest.allakhazam.com/db/item.html?item=45265