One of your requirements is: "Motor needs to be reliable in that it has to run continuously 24/7/365. BUT you have not described how you are going to ensure the entire system will be powered so this service can be obtained.
Curious, what happens if the system suddenly stops one year form now?
I'm not quite sure what you're asking. The system will be stand alone and based on the stepper motor that is needed will dictate the power requirements. There will be no other outside influences on what controls the system other than the poteometer or buttons to dial in the RPM.
Some more details: I'm a saltwater reef tank hobbyist and to meet the needs of calcium in the tank to grow corals, we usually employ a calcium reactor which is simply a container with calcium rich media and by lowering the pH inside the reactor (by pumping in a metered amount of CO2) the media is dissolved inside the container and then a separate pump slowly pushes tank water inside the reactor and pumps calcium rich saltwater into the tank at a very slow rate. normally this procedure is done with a small 120vac power head pump but the flow rate varies and the accuracy is far from providing stable water parameters.
People have started using paristaltic pumps to meet these needs, but these stand alone paristaltic pumps aren't cheap and I thought this would be a great project to DIY.
If the system stops after a year of operation the tanks stability will be compromised, but I will be alerted by a sudden change in pH and I will be able to take immediate action to solve the problem.
Forgive me if I did not answer your question, hopefully this will give you more insight.
For that sort of reliability you need a motor without brushes. (preferably is fully sealed and uses
fully sealed bearings).
In a clean, dry environment without much dust such hermetic requirements can be relaxed. If
moisture / leaks are possible, a "food-grade" hermetically sealed motor is probably a wise choice,
though more expensive.
Steppers and brushless DC motors are possibilities, stepper being more power hungry but simpler
to program (no encoder needed).
And clearly it needs enough torque for the pump load.
The exact metering of the fluid by exact revolutions is why I chose a stepper motor rather than a BLDC motor. Humidity will somewhat be a factor the room in which it will be operating has a humidity control to keep the overall humidity at 20%, and the calcium reactor will be far enough away from salt water that it won't get wet or splashed on. and of course another added layer of protection will be that the entire unit will be sealed inside it's own container.
Are most stepper motors designed to run continuously or is that a special feature that I have to look for?