[yellowapple]

"And the machinery to gather them"

That machinery can be sent gradually and ahead-of-time.

"Just because the raw material is there doesn't rocket fuel make"

No, but it means that such fuel is at least possible.

"I'd be hard pressed to say that oxygen is plentiful on Mars"

The current understanding is that there's millions of cubic meters of water ice on or near the Martian surface (with even more suspected further beneath the surface). Sounds pretty plentiful to me.