Good Evening, Stregone,
The 80mm El-Nikkor should serve you very well for 6 x 6; these days, it's a buyers' market for quality enlarger lenses. A similar Schneider or Rodenstock would be fine also.
Black plastic will work fine, but you'll almost certainly need multiple layers to avoid leakage. For a door, start with good weatherstripping. Add more black plastic hanging from a rod at the top of the door frame, and you should be good to go.
Good easels, such as Saunders, should be fairly easy to find, again at bargain prices. Getting one locally will save on shipping costs (good easels are a bit heavy and bulky), but you should be able to sense the market by checking E-Bay listings.
Check the APUG Forum under various key words which you should easily be able to imagine and the photos of lots of darkrooms which have been posted in a lengthy thread here.
Konical