Answer to question 1. Many times you can use 1 server so long as you have these things in your server. Radius server software installed & a billing & managment program installed on your server. You can have different work with different destinations. There is a limite on destination which depends on the gateways & gatekeepers you use.
I am unable to answer your questions to 2 & 3. I will be happy to answer them, but I would need I rephrased or more in depth details so I can answer your question. You can email me at email@example.com or you can see me on windows messenger firstname.lastname@example.org