Everything would depend on efficient utilization of the bluff. That is how Sgt. York achieved his coup against the Germans. The Marines would have to move fast to establish their authority before the Romans regrouped from the shock and awe.After that, everything would depend on the knowledgability of the Marines in metallurgy and chemistry to develop their own logistics.
The Marines would also have to conserve everything while they worked on chemistry and metallurgy. I would not bet on them producing a comparable gunpowder on their own quickly, but a full combat load of ammo could last a long time if limited to sniper-quality shots. The officers would be college graduates, many of them with some engineering and almost all with at least calculus and modern chemistry. Throw in senior enlisted who reload, and they might have some potential to resupply, over time and as they learned the right Greek/Latin words for what they wanted.
As for the tanks, I’m not betting on horses or slaves dragging them around. They’d be great as fixed defensive guns, but I don’t think maintenance would keep them mobile for very long. Depending on the propellants in tank guns, they might even be better off cannibalizing the powder from tank shells for reloading small arms - if they could figure out the primer issue. That’s after an initial shock and awe that would be epic, of course.
If they saw themselves as Green Berets embedded with the local army (Roman Legions) as force multipliers, their initial supplies would matter for a VERY long time though. Any Roman army with a squad of riflemen would have very high moral and face an army already expecting to lose almost from the start (if the Marines could avoid being betrayed, surrounded, etc.). Classic Roman attacks, but with the certainty that the defense would collapse at the point of attack (as officers and men in key positions dropped due to almost unstoppable sniper fire) would be powerful.