I've been doing a search on rustproofing and have now decided to go with Rust Check. Do a google search on it for a location if you're interested, cost for a car is around $120 to $130.
If doing it yourself I'd go with an oil spray that contains lanolin. Trouble with DIY is you do not know where to drill holes and more than likely have the equipment to spray the material inside the body panels, doors are no problem but what about rocker panels, door posts, back near the trunk and around the gas filler cap, I think you're getting the idea.
For a $125 approx I'd let a pro that has experience and knowledge do it.
Just my 2 cents