Welcome to the Holacracy ecosystem. The role of the Facilitator is to safeguard the process. The more he can focus the better. So you are right that having full time facilitators makes perfect sense.
The Facilitator role is an elected role from the people in the circle (Core Circle Members). This is how the constitution deals with it. But in practice it is also stated that learning to play the rules from an experienced Facilitator is the best way forward.
So, if you have the opportunity to have full time and experienced facilitators that would be of a great help the teams to learn the rules of the game faster.
In my opinion there are no other specifics than the election part in the Constitution that define that the Facilitator must be from the Circle.