If you know what your doing, then i say "It's well worth it to do it on your own"
You don't need to buy another drain plug. I'm still using the original drain plug and washer for my HHR. I've got 96,000 miles on it without a single drop from the plug. I bought the car new.
Is the IGN-3 really needed? I don't see why you can't pull ACC power from the same fuse as the radio, that's how I wired up my dash cam on my HHR.
Now I could be wrong here, I've never worked on a car with start and stop.