Just speaking from an objective standpoint here. If the rusted through spots are no bigger than an eraser, go to Harbor freight and for $150.00 buy a mig welder. you won't have to cut the body to fill them you can just put a weld in it using the welder to fill the hole. Grind it smooth and its done.

As for throwing it together to get a profit, I can understand that but realize that this isn't an 89 Honda you are dealing with. If someone comes to look at the car I am sure they will be toting a magnet. If they see signs of bondo all over the place they won't buy it. To them even if it looks good on the outside no telling what it looks like under the paint. They would rather move onto an old faded out, surface rusted up car atleast then they know what they are getting into, and not stumbling on it as they go.