(1) no, a GD exhaust wont bolt up. What muffler does the exhaust already have because a good way to keep the performance exhaust but give it street manners is a nice straight through oval muffler. Will make it civil when driving around but still a nice noise at wide open throttle.
(2.1) A GD dash is not for the
faint-hearted, there are a few Australian threads on doing the swap but mostly American one which are not ideal as there are obvious differences.
(2.2) If the dash is in reasonable condition maybe consider a nice dash mat can clean it up, and also a later model steering wheel makes the car feel less aged.
(3) Could be a number of things but sounds like a possible boost leak.
(4) Could be a bad strut / coilover, maybe the top mounting nuts are not 100% tight, maybe a sway bar mount is not tight.
(5) No, sorry.