Reglet is much more difficult to manufacture yourself, so you're probably better off trying to buy some from your typehouse. Clean your furniture and reglet after printing and store it carefully and it will last you for generations. Leading is the material used to provide spacing between lines of text. Buying your leading in bulk is obviously less expensive, but you'll then need to acquire a lead cutter. A 10 pound package each of 1pt and 2pt leads will probably be sufficient for a good long while.
Slugs are just like leading but thicker.
The type is precisely cast but, as you tighten your form into the chase, some letters may rise up a bit. At this point, just prior to putting the chase into the press for printing, you stop off at the stone and plane your type down to level. Wipe the surface of the stone clean and lay your chase on it. Lift one corner of the chase and try to wiggle the lines of type. If anything moves at this point you'll need to figure out which lines are short and add more spacing to them so all lines are exactly equal in length.