I don't know how to bargain, so, I went to the place that Archengager mentionned and got a Xite for ~3900rmb (80km, 60km/h), the larger one.
It got stolen after two months. Now I have a Lima (120km, 45km/h, modified to 60km/h, 80km) bought for ~3500, it looks like it's just a tiny bit of lower quality compared to Xite, TailG, but it does the exact same thing as the previous Xite for 400rmb less. I'll see if it's still going to me mine after 2 months.
Just get a cheaper second hand from classifieds.