I'm fairly certain that knowledge of modern engineering, medicine, weaponry, etc, would make it trivial to defeat the Romans, provided you had enough to people helping you. The hard part is figuring out how to gain control of people and I think there's very little we know now that would help. Probably the best bet would be to arrive somewhere there's relatively easy access to gold (provided this doesn't violate your first rule) and simply pay for workers and mercenaries. As it became obvious that you know what you're doing, I think they'd become more liable to follow you too. But even then I think the odds are not great that you could get enough people involved.