There are no hard and fast rules with regard to this kind of stuff, so its general info only.
1. You don't need to have a full proposal written down (although some will expect this) to contact potential supervisors - just a very clear idea of what you want to do. You will need an analytical framework and a series of questions you want to address as a bare minimum.
2. If you are applying for PhD funding often you will find that supervisors will help you refine your proposal especially if you want research council funding... but this is not necessarily true.
3. Most people I know applied for half a dozen - I applied for 4, and got 4 acceptances, but only 2 offers of funding