First of all, the five-tier system for strata schemes in Victoria is explained HERE. Just click on the link and the factsheet should download to your device.
To answer the question, for a tier 2 scheme (51 to 100 units) a strata managing agent is not a legal requirement but it might be a good idea.
The fact that you are asking that question suggests to me that your scheme might benefit from a little guidance.
As to a “model” if your scheme has a lot of issues and squabbles between neighbours, then you should seek out a contract that is all inclusive with a one-off payment for all services.
If your scheme is settled, you might take a less expensive contract that allows for charges on the basis of work done above and beyond normal duties.