The cork check is basically a piece of metal or plastic that goes in front of the grip to give a transistion from cork to blank. Mostly cosmetic but will help keep moisture from getting under the grip if sized correctly. As far as reel seats and their hods. Down locking do not have recessed hoods where uplocking do. The recessed hood in an up locking seat is the piece of metal that the reel foot slides into under the cork grip. On down locking seats there is usually something similar to the cork check where you have a piece of metal that transitions the cork grip to the wood insert. For thread, if these are your first rods, use a nylon size A thread and flexcoat or flexcoat lite. I would head over to www.rodbuildingforum.com
and have a look around. There are a ton of great guys over there that build some great looking rods and are more than willing to help out. There are also several tutorials showing you how to do things.