These are useful for holding pieces together while glue sets. They are cheap and come in different sizes.
These are used most of the time when assembling kits. these are fairly cheap and 4 seems to be a good number to have, unless you do this often.
You will only need these if you are assembling one of the larger kits, such as Merlin's Casket or Sarcophagus.
Wood glue is a necessity when building these kits. Try to stay away from superglue, basic wood glue is all you need.
I find these helpful to fine tune certain areas that are sticky. these also help if glue gets where it is not supposed to.
This will smooth your edges and prep for any finish if you decide to paint, clear, or use patterns. 150-180 grit seems to be the best middle ground.