I am restoring a La Cimbali M20. There are several gaskets that need to be replaced in the group head and heat exchanger. O-rings are available in many different forms, and I would like to hear some opinions on this topic:

What material is best: silicone vs. teflon vs. viton vs. paper vs. buna-N ?
Which ones are food-safe?
Will any of these change the flavor?

Another question specific to my machine:
One of the o-rings that I need is available here for $10 : http://www.espressoparts.com/G_191

The description on this site states:"Used in combination with the paper C_88 or Teflon C_315 when used on the heat exchanger, but not needed with the heating element"
Is this necessary? When I took it apart, there was only an o-ring (which was dry and hard as rock)