Your best bet is to do A LOT of reading in these forums and on chevellestuff.net.
I will give you a couple tips to start you off.
1. Stay away from classic car dealers unless you dont mind over paying from someone that knows little about the car they are selling.
2. You are better off buying a completed car as opposed to a car that needs restoration. It will be cheaper and your FIL can enjoy the car immediately.
3. If you insist on a real SS then do your homework on identifying a real SS. Plenty of fakes out there.
I couldn't have said it better than Brad does here^...I agree. I would only add one thing to his #2 point: I agree on the part about buying a completed car being easier to start with, but you have to closely look how the car was put together as far as any custom work or custom parts. If the car was previously restored as close to factory specs as humanly possible, then be preppared to spend BIG dollars to purchase the car, (which is ok if that's what you want).
But if the car was customized with aftermarket specialty custom parts, look at it very closely to see how things were done and how parts were installed as well as what parts were used, and exactly how they're different from the original factory OE parts. Because lots of guys who customize old classic cars like these, will do things the way only they wanted, which might be so unique it isn't how you nor how your dad, nor how anyone else would want it. And if it's a major part in question, or the frame or chassis was modified to install the part(s) in question, it might be difficult, expensive, or even impossible to undo what was done.
Here's one of the most obvious examples I can give you: ALL of the aftermarket custom frames for these GM, A-body cars I've ever seen advertised in catalogs, not only have extra steel in certain places to beef them up and make them more rigid and more sturdy than the original OE factory frames were, but they also have main frame rails that are shaped very different, and therefore have a built-in lower stance which PERMANENTLY LOWERS the vehicle ride height, and also makes it impossible to use stock replacement tail pipe routing over the axle tubes the way the Chevelle had from the factory, since there isn't room for them with these aftermarket frames.
But the sales people won't tell you that. So if you're looking at a Chevelle which has one of those new-fangled fancy frames, you will not be able to have a factory stock ride height, nor use tail pipes. Instead, you'll always have to have the exhaust empty out underneath the car, or have some make-shift turn outs that route the exhaust on the side of the car in FRONT of the rear tires.