Let me elaborate, If the Impala needs too much work for the resto to be done, and the car to be safe afterwards, then follow my suggestions.
However as most older chevs are not unibody cars, you can get or fabricate most of what would be necessary to restore the Impala. But if you have to buy all the body panels, and fabricate parts of the frame and floor, you're getting into some big money. A $50-60,000 resto on a car that you have no real history/attachment with, can fast become a "money pit" restoration.
If, however, you grew up in a 409 Chevy, and have lots of fond memories then you can't spend enough to restore it.
Also, insure that the motor is restorable. If it has a cracked block, or has shot a piston thru the side of the block, etc. It will just dis-appoint in the long run.
I would do the same if I had a shot at the T-Bird I grew up in. Any other T-bird resto, I'd start with the best car I could find.
In the case of the Impala, you are in rare waters, If it isn't hugely rust and a cool motor, go for it, and follow LK's advice.