Am sort of sad cause i would love to go China. Now they send me an invitation letter and work permit, and want me to get my Z-visa and leave asap. Can i look for others schools, or is it too late once they issue you those papers? I have the documents in my place, and have not applied for the Z-visa yet.

A bit of a no-brainer in my view.
You are morally obliged to take up the job offer.
If you wish more detailed advice on what to do when you get there, and on the attractiveness or otherwise of the whole contract you've signed, and the cost of living there, and house rental norms there, you should provide us, the other readers besides yourself, with your e-mail address and/or your Skype contact details.
Actually, I have taught in Changchun for one semester, so I have background information that would definitely be useful to you.