You didn't say which car.  On an E46 ('99-2004 3 series), yes, you need
the entire lower control arm as the ball joints are pressed in and not
separately replaceable.  There are aftermarket lower control arms with
replaceable ball joints, with which I have no experience.

Yes, a competent mech should be able to change them.  I did.

Depends on what kind of car it is.  Some of them have joints that can be
pressed out and replaced, others you have to replace the whole assembly.

On some of them, it can be cheaper to replace the whole assembly anyway
if the cost of the bench labor is high enough.

Probably, although you want someone who is familiar with BMWs because
it's all a little different than a Ford or Chevy.  It's not a hard job,
though, you could do it in the driveway if you were patient enough.
--scott

What Scott said.

The Lower Control Arm holds the lower ball joint, and it locates the
suspension (Caster angle) via the bushing at the end that is bolted under
the passenger side or driver side footwell, as appropriate for the side of
the car.

Ball joints are not indistructable, but they also seldom wear out. The
control arm bushing is another story. It is distructable, and wears out
often. It is easy to unbolt the control arm in its entirety and replace the
assembly -- including the bushing that wears out and the ball joint that
does not.

Odds are good that you will not screwup the alignment either.

Let me clarify.

If you have been driving the car for a long time and never had it aligned
because the tires wear okay and it doesn't pull to the side, then you can
replace the lower control arm and the car will remain straight. On the other
hand, if the car was pulling to the side and you had it aligned (they
adjusted stuff), then when you put good parts back on, the adjustments will
be wrong, and the car will pull again. Well, it might pull again.

If it never pulled before, it should not pull because of the new lower
control arm.