It's better to run off another amp.  But it's always possible to run
the speakers off the head unit and the subs off a separate amp.  Just
don't try to run the subs off the head unit.

That depends on a lot of things.  If I hurry and there's no problems, I
can usually do a complete install (HU, amps, speakers, subs) in a day
as long as I don't have to make the box.  Here's some pointers:

Don't hack the wires in the dash.  Pay the $15-$20 for the wiring
harness at best buy that matches your car.  This way, all the wires are
labeled and you can make the wiring harness outside the car.

Don't twist and tape wires.  If you can't solder them, use butt
connectors.  You'll want some red ones and some blue ones.

Buy your wire at home depot at the big rack where they cut the wire for
you.  It will work just fine and cost a lot less.  Same for the
fittings that go at the battery post and the grounding point.  Some
will call this heresy.

The bolt for your grounding point needs to be *really* tight.

Put a fuse near the battery!

If your head unit has 2 or 3 pairs of RCA outputs, but you only need
one, run the RCA for all of them anyway.  It will save you a lot of
headaches later.  Don't forget the remote turn on wire.

brian