Contacting professor for PhD in different research area than past experience: do I need to prepare a research proposal before first contact?
the answer depends a bit on the culture of your country and even on the individual professor you are adressing. If you would contact me, sending an e-mail containing more or less the question posted would be sufficient to get an invitation to talk about potential research topics and about how to proceed. 

At least in my position, the answer is more dependent on whether I have an idea on how to finance your position - so if you have ideas about this (grants, ...), you should include such information.