If its surface rust on the outside your ok. You can pretty much take care of that. What I would be concerned about is if there is any rotting of the frame on the inside !!
I looked at around 8 different jeeps in a month and a half's time which all looked somewhat good on the outside of the frame but were rotting out on the inside. I finally found the one I just bought with a solid frame and body.
You really need to lay down on your back and check inside the openings of that frame before you settle on it !!!