I wonder how I can obtain the items needed and if it 's actually possible, since the head is DK only (I mean ofc it 's possible since somebody else has it, but I have no idea how they got it)
The head isn't DK only per se, it's only Q reward from a quest only DK's can do. The item itself has no class restriciton.
Dreadnaught items can be obtained via coin store as well.
Yes you can. You just get raid group and clrar dungeon. You can also reserv item that you want, but make sure to note it in your raid advertisement, cuz getting 30 day ban for TBC purple item is sad
Few level 80's for the TBC raids. More if you wanna do full clear of Karazhan for the last boss (as the chess boss needs more players).
Karazhan questline is needed to complete, or at least by one player in the raid, in order to summon the shield dropping Nightbane.