400 rpm sounds like a reasonable cutin for a 4 ft blade diameter if it is for a 12v system? Rated Voltage(V):28 is a bit odd - usually 12/24v Net Weight(KG):17 - is pretty heavy - The Ametek 30 is about 3 Kg...
Maybe 13v will be achieved at 185 rpm - great This is the problem I think dealing with China, communication - obviously language and maybe the time difference.
Maybe you could contact them directly with your inquiries and maybe request a sample. Be careful though, there are certain laws and fees that may need researching - I don't know perhaps you are aware of all the pitfalls?