Yes, I pour concrete, and do it as a profession. $4/sqft. in my area for the time being. Just like a paint job on a car, prep work is key. You need to have a solid base, and make sure your grade is consistent.
Rebar is highly recommended every 4 ft for light duty, every 2 ft for heavy duty usage: semis and tractors I would recommend 2 ft.
Whether you can do it yourself depends on quality and quantity of help, and equipment. If you have good help, and enough help, you'll be fine.
You do not have to pour the whole slab in one pour. It can be broken down into 2 or three pours depending on your help situation.
What finish are you putting on it? Broom or trowel finish? Trowel finish requires more effort and skill, or you end up with a big mess if you don't know what you're doing.
Decide what you want, whether you have the necessities to tackle the job, then decide if it's worth the $8k to have it done. If you have the resources available to do the job, it can be for less than what you've been quoted. If the resources are not there, it's best to pay the extra amount.