That's insane. Most places will change the oil and filter for you for around $15 or $20.
Are they going to do a whole tune-up or something too, for that price?
If not, changing it yourself is easy, if you have the tools (inexpensive) and the desire.
I don't know about the differences and tradeoffs, so no comment about this.
The ones that go with the engine. :-). Go to a local parts shop and ask them for the spark plugs for that car. The ones for my BII were $1.50 apiece. Or just have the service station doing the oil change do that job as well.
I hope this is your second oil change you are talking about!
You're welcome.
-D