Personally, i'd probably do it myself, but i have the time and it's the type of thing i like to do. However, i wouldn't try and get out of it on the cheap by levelling and throwing a few
seeds around.
If you decide you're gonna do it yourself, do it the same way the experts are gonna do it, or you'll be disappointed. Then in 12 months time you'll be digging it up and getting the experts in anyway. Do it once, do it right.
Do the soil test, add the lime, remove the old grass, etc etc.
To be honest, in my opinion, if you're not inclined to doing it properly, pay the bucks and get it done by experts. The quality of a job should last long after the price is forgotten.
Get a company with a good reputation. Ask them to direct you to jobs they have done previously and go look at a few.
700 bucks is a lot of money, but if the job is a good one then so what. The peace of mind will be worth it. 300 bucks wasted is a heck of a lot more in my opinion.