The best pencil kits are, in my opinion, the ones that use a Schmidt pencil mechanism. Example would be the Perfect Fit Convertible pen, El Grande Pencil, Gent Jr pencil etc.
These mechanisms were also available many years ago in European style kits from Woodcraft, not sure if they still sell these.
Personally, I make only the PFC, the charm of this pen is that I can change on the fly from a ballpoint pen to a pencil. So I just buy a number of 0.5 mm and 0.7 mm pencil refills to be prepared.