As far as I know, the only two "totally legal" ebike models in China are BMW C EVO and lifan Le E. Why they are legal is because they need a plate and license of motorcylces.

The BMW C Evo is about 99800 RMB and required a yellow plate.
The Lifan bike is about 7000 RMB and required a blue plate, lifan said that it can get the speed over 50kmh and run over 120 km, if you want this just contact the local lifan dealer, lifan is a Chongqing manufacter so it must be popular in Chengdu.

But the above 2 cannot match your requirement, If you just want a ebike without a legal plate, you can just buy a normal one and change the computer and battery, or simply buy a modified second hand ebike, there's lots of people interested on moding ebikes in China.