The bit about housing is tricky. If they cannot provide housing for free, you shouldn't go there and look somewhere else. You may be expected to pay a lot of money up-front which you will never see back if you get private housing.

Also, be very wary of working illegally. Shenzhen is a city where immigration is tough and always ready to hunt down illegal foreign workers. So, make sure that you will get a new residence permit and work visa before you sign up with them. Ask them for a copy of their license to hire foreigners. If they cannot or are not willing to provide evidence, give them a big miss. Something must be wrong then.