You need a 10-speed chain for this bike. You can find them for eBikes here: https://www.galaxus.ch/de/s3/producttype/velokette-806?filter=t_7318%3D341203%2Ct_7502%3D6016

I'm 183cm tall and the bike fits just right when I adjust the saddle to its tallest allowable mark. As already said, it depends on your leg length. My guess would be 173cm-183cm is a good range.

I ordered without assembly and still delivered assembled xD.

Yes, the assembly is relatively simple. Depending on the product, only the handlebars, saddle and pedals need to be installed.