I am not an attorney, but I have graduated from law school, and have been involved in several lawsuits involving classic and muscle cars. All of which I was successful, and got the buyer tens of thousands in damages. However, the lawsuits in which I was involved, dealt with vehicle misrepresentation such as the car was advertised as a numbers matching Z28, which it was not, or the car was advertised as a numbers matching, but the engine had been stamped to look like it was from that car, and all were from specialized car dealers, not a seller from the general public. I got Jerry involved on one of them, as it was a 69 Camaro, and his report was golden, and we settled out of court.
In the case where the seller is a dealer specialising in muscle cars, he is held to a much higher standard than just a regular person. A person on the street is only held to the standard of a typical person, and not as an expert. Therefore, he will get the benefit of any doubt, and what he said to you to make the sale, may amount to only "opinion" or "puffing".
There are several things to consider. First, where is the seller located? If he is out of state, you may have to sue in his state, and not yours. You may be able to state that he availed himself to your state by placing an advertisement that was meant to be seen by you outside of his state, and if you get the right judge, that will be enough to establish jurisdiction. I found a case involving an ebay sale, which hailed the seller into the buyer's state, which made it easier for the buyer and difficult on the seller. Also, just because he said the car was in great condition, may not make him liable, as again, it was his opinion, which is different than stating as a fact, which is tough to prove in a courtroom. Even if he made statments of fact, you have the burden of proving what was said, which in darn near impossible. You may be limited to what the ad stated, and anything you could get him to say during depo, if the judge even allows you take his depositiion, as in small claims court, the rules are more relaxed, and the defendant could argue undue burden, and prevail in halting a depo of himself. Next, you have to prove what he actally knew. You would have to convince the judge he knew about the rust under the top. All the seller has to do is play Sgt. Schultz, and say he knew nothing, and he will be believed.
Without knowing the whole story, and only going on what you stated here, I will say that your likelihood of being successful in court would be low. And I doubt you will find a lawyer who would take this on a contignecy. Which means you can easily find a lawyer who will take your case, but he will want a retainer. And it will take a year at least, and he will keep asking for more and more money, as the lawyers fight it out on every stupid detail. Long gone are the days of going to trial quickly in civil suits. Just as doctors have to order every single test in order to cover his butt, lawyers have to argue every single nuance, regardless of the chance of success, just to cover their butt...and to pad the bill as well. It will take 6 months to establish jurisidiction, then there will be summary judgment hearings which is what the lawyers do to get the case thrown out on a technicality over the stupidest reasons...and on and on. No matter how gung ho you are going in, I assure you, you will be dogged and no longer care long before it is over. And if you have an attorney, the money spent could have gone a long ways in fixing your car. Also, there is no guarantee of getting attorney's fees if you win, unless it is given by contract or statute. I have seen cases over $200 that amounted in over $20k in attorney's fees, that the plaintiff had hoped to collect as well. The judge agreed he was owed the $200, but gave not fees, and it cost him. Lesson here is don't let pride get in the way of common sense.
However, if you want to do something, and you believe you have jurisdiction in your state, you could sue in small claims court. It's much easier, you don't need a lawyer, and could actually be a great learning experience. Depending on your area, you are limited in what amount you can sue for, such as $5k in my state here. The rules are a little relaxed, and the judge basically will typically attempt to advocate for both parties, give legal advice, and not hold you tot he letter of the law and proper procedure, as they surely will in a higher court. File a suit, basically telling what happened and establish your cause of action, which is the legal reason you are suing. It would be typiclally something like breach of contract, but may include other causes of action, depending on the facts of the case. Bring in the ad, and any other evidence you have. Get your body shop guy to testify as to the actual damage and cost to repair, and if so, an expert on what the car was really worth when you bought it. You will have a 50/50 shot I bet. However, it is "buyer beware", but a consumer does have some protections, just not many...as the seller is also protected as he did not hold himself out as an expert here, and will also get the benefit of the doubt. In fact, the seller may not say a word, and if you can not prove your case, he could win. Small claims will cost you a few hundred bucks, whereas an attorney, $5k-$10k easy. You get to tell your side, and watch the seller explain himself in front of a judge. That may be worth it. And, if he doesn't respond, you can get a default, prove your damage, and get a judgment. Collecting on that judgment is a whole other issue.
Hope this helps.