1- No you don’t need to be an architect but It’s a bonus, of course. You need to be able to read some plans at least and be able to visualize a 2d plan into a 3d space.
2- 
3- Usually people charge per amount of stills, movies or executable. You may charge per hour when a client ask for changes. Extra work paid by the hour. For example, a video could be sold 100$/sec of animation. You can charge x amount for the modelling, X amount per rendering, X amount for a complete .exe, etc.
4- Some people will offer 1 or 2 revisions for free, other will charge per hour for it.
5- Usually the client will not have the source file unless he is willing to pay. It’s a better idea to keep the source files for yourself tho. That way they cannot turn around and give the next rendering job to someone else because they got all the assets!!!
6- Top studios like MIR charge around 4000 euros per image. (source : chaosgroup lab podcast)
7- Don’t go on freelance website it’s depressing and people are offering renderings for 20$ and they are destroying this industry! Aim to be the best and offer an outstanding customer service and you will get clients.
Bonus : this whole forum is specialized about the business of architectural vizualisation. http://forums.cgarchitect.com/227-business-visualization/