UnRAID is likely the solution for you. It’s not raid, it’s JBOD (Just a bunch of disks) with a separate parity drive to give you fault tolerance in case a single drive fails. You can use any combination of drive sizes and models and expand over time. Only rule is the largest drive has to be your parity drive.
It’s got a lot of other things going for it that make it an especially good “First NAS” solution.
I’m going to go out on a limb and say they probably just want a comparable solution to Ollama.