If this were a real-world project, assuming that a person is placing the container on the conveyor, it would be safer to have a start button. That would force the person to get his hands out of the way of the conveyor before it started moving. You should always have some method of stopping any system so that when things start to go wrong, an operator can shut down before they get worse. The details of how to go about designging safety into your system depend on both the system and the amount of risk involved in its operation.
Since this is an academic exercise, it doesn't really matter whether you start the conveyor as soon as the container is placed in front of the sensor or whether you wait for a signal from the 'Start' button. However, it's best to develop good habits early rather than have to 'unlearn' bad habits later. Include the Start and Stop buttons in your design.
As to whether a pushbutton should be wired normally open or normally closed, you have to consider what the button does and how it might behave when it fails. You can write your PLC program to handle either choice of contact. Think about your stop button. Its function is to stop the conveyor. Two of the ways it might fail are when the electrical connection between the button and the PLC gets broken or when pushing the button fails to change the state of the contacts. If it's wired normally closed, and the electrical connection between the button and the PLC is cut, from the point of view of the PLC program, that's the same as pushing the button, so the conveyor stops. If it's wired normally open and the electrical connection between the button and the PLC is cut, you can push it as hard and as often as you want, but the PLC will never know, so the conveyor keeps on running. Those facts argue for wiring the stop button normally closed.
For the other mode of failure I described (pushing the button doesn't change the state of the contact), it doesn't matter whether the stop button is wired normally open or normaly closed.
Big honkin' disclaimer
The preceeding discussion is not intended to be interpreted as claiming that all stop buttons wired to PLC inputs should always be wired normally closed. While that is common practice, every situation is unique and should be evaluated on its own merits. The discussion is intended to illustrate some of the factors to be considered.