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Idle Behaviour, manifolds, O2 sensors

My '85 5000 CS Turbo is experiencing a rough idle. Basically, the car will
run a bit rough, but the main problem is when I rev the engine, the revs die
down and fall under the normal idle to the point when the engine will shake
and stumble, but it doesn't stall. I just had the idle speed valve replaced
with a used part. I know this kind of thing can be caused by a myriad of
things, but I have a few questions I hope you guys can answer.

1. What are the syptoms of a cracked exhaust manifold? The car runs fine
when it warms up, but it is just when I first start it up or after I start
it after it has been sitting for a little while.
I am getting some exhaust coming out of the engine bay and can smell it in
the passenger compartment, but I have inspected all the pipes and fittings
and everything seems good. Could it be the turbo and/or a bad manifold? The
turbo still reads 1.4 bar max boost.

2. In a normal, working car, what would happen if you unplugged the O2
sensor at idle. I can do this and notice no change in the problem I am
having, so I think the sensor might be dead. I lack the volt-ohm meter to
test it, so I was wondering what you guys thought.

3. Lastly, how much does a good engine re-build run? I know it varies, but a
rough idea would help me avoid being ripped off. I know it needs new valve
stem seals at the least.

Any help would be greatly appreciated.