2002 Subaru 2.5 Outback

Hey Ya'll!  
I'm a plumber by trade and a fairly decent
mechanic(Overstated) and have an issue.  The
check engine light has been on for a while and I
used the Mariotzi trick with the black tape. Well, I
took it in and originally said the cad. Converter was
bad so I had it replaced. Now(not rote after but
within a couple months) it overheats constantly
and the Autozone computer says its misfiring on 3
cylinders. In this circumstance, 3 out or four IS
bad. I noticed that the water pump and the timing
belt at married. Also noticed that the heat only
kicks in when I gun the engine on the interstate.

The question is..  Should I replace the water pump
and get at the, Potentially , obliterated timing belt?
It's cold oud and dont want to make any rash
decisions.

Any help would help greatly!
Thanky!!