1. Yes. I spent last summer (about 3 months) as a courier in Seattle and I liked it alot. The money wasn't great. It worked out to about $90-120 a day, sometimes more, sometimes less, depending on how many packages they give you.
2. I got alot of enjoyment out of it, but I was also glad that I only did it for 3 months. I think it'll depend on the person.
3. No. My bike got about 50 mpg. I use relatively soft tires, because it always rains in Seattle, but they're not super expensive. Just don't crash, because THAT's expensive. Also, don't ride a brand new bike, obviously.
4. Depends how much money you want to make.
Make friends with your dispatchers. Where I worked, they determined my paycheck more than any other factor. And of course, drivers are idiots, you are invisible, stay out of their way.