You can get the Marian reforms as early as 242 BC, once you have triggered Polybians. You do need to own 90 settlements, which is a bit beyond what most players would have at that time.
Not all provinces can build Marian units. If you venture into the steppes, or deep into Asia, you will find there are actually areas where you can't get them. Use recruitment viewer in case of doubt.
As an aside, I think the battlecounter has been disabled for the reforms. So there would be no requirement for you to have battled at least 250 times.
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