The heater bypass pipe is steel, on the water pump end, there is a rubber
seal, which should be replaced, the end of the pipe will be corroded to a
certain extent, but will probably still be good enough to seal again once
the rubber is replaced.
The new pump should come with a gasket, a seal for the top, and a seal for
the bypass pipe. The bypass pipe is held on to the back of the pump by a
small nut and bolt, the head of the bolt is a hex drive of all things!

If there is anything in your cooling system that might damage drive belts,
then is already trying to escape through all your coolant hoses with
extreme predjudice, in other words, don't worry about it!

BTW, when removing or replacing your water pump, be careful not to lose any
nuts or studs into the timing belt cover, they are a real pain to get out,
as some folk on here could testify, there a few cunning tricks to
retrieving them, some of which I would never of thought of, but I've been
lucky enough (so far) to not need them; just to be safe you could stuff
some rags down into the cover, thus blocking it up. Also when replacing the
pump, notice that some of the holes are elongated, this is to allow you to
use them to fit the pump lower on the block initially and loosely, then
lever the pump upwards into place, then pinch up the fastenings, the other
bolts will line up correctly when the pump is positioned properly.

You could also use this opportunity to check on the condition and tension of
your cambelt.

Good luck, Ken