Boiling the water drives off the chlorine. The bicarbonate will interfere with the fermentation process by buffering the pH to too high a level and will reduce the effects of other beneficial ions in the water. Boiling will precipitate the calcium bicarbonate as a powder at the bottom of the vessel containing the water or as a scum floating on top of the water. Either skim the scum off, or decant the water from the powder. These are available at most homebrew shops.
Kits generally contain malt extract which is already hopped, or come with loose hops.
With already hopped kits you have less work to do and the levels of bitterness should be constant between brews, however you have less control over the finished product, so should you want to alter the bitterness levels, you would need to move to kits which come with separate hops. If a kit does contain loose hops check these closely for quality. Look at the base of the hope cone. You will see a powder which should be yellow or amber in colour. If this is brown then the hops are old and are of poor quality.