Your entire post is a really good point, but if we're allowing all sides to have access to their complete arsenal of infantry weapons then I still feel that the Marines are forgeted.
If I have 60,000 Roman Soldiers (who depending on definition may not even be Italians) all equipped with bows and arrows, then I can unleash such a volley of arrows that no Marines would survive, even if they took out thousands of Romans.
If we play in a real environment it only gets easier for the Romans to get into a perfect range.
I feel like the Marines would win in most battlegrounds.
If it's on an open plain, I feel like it would go to the Marines. Compare the 800-1200m range of many of the Marine's weapons to the 100-200m range of the bows and arrows used by the Romans. In a wide open space like this (say, the semi-arid regions of Spain, the deserts of North Africa or Palestine) all the marines would have to do would be to kite the Romans, plinking off their soldiers and constantly moving out of the range of their bows. If you want to get real devious, the Marines could lay down mines like Claymores in order to further delay and attrit the Romans.
Similarly, I can't see the Marines losing in a mountainous region either. All the Marines need to do is capture and hold the high ground, and they can fire down on the Romans with impunity.
As for the arrow thing, I don't think it would work very well. Volleys of arrow fire were good at thinning out ranks of tightly packed troops in a formation, softening them up for melee fighters and heavy infantry to finish them off. However, the Marines, due to radio communication, would be able to spread out and fight from multiple positions with no loss to their tactical ability. Once the marines have split up into platoons or squads, archers have to aim at individual targets instead of areas and formations. And it's obvious, but bows are very poor when aiming at an individual target at 100-200m