In today's market and the prices being sought and paid for some of the original or near original cars, I first want to know the history/lineage of the car - from owner to owner over the years. If the car came from any place "up north" where they use salt on the roads in winter, and now it resides in the "south," I would be suspect that that car has, or may have, hidden rust and I would crawl all over it with a microscope/camera. If it's home was anywhere near oceanside, where salt air can corrode as quick as northern salted roads, I would want to know. I would want to know if the car was always garage kept. Rain carries chemicals & dust particles in it that settle under moldings, lights, and inside body panels only to later rot out or bust through - even though it was a low mileage original car.
If I see undercoating I am always suspect. Unless you can document and verify the original owner had this done at the factory or dealership prior to accepting the car, then I feel something is being hidden. Hearing that the owner wanted to preserve his car so he did the undercoating himself is not good enough for me.
I could go on as other will hopefully contribute on things to look for on a car that is still claimed to be an unmolested and original car. Original cars are nice and can be an investment, but for me I want a driver and not one I have to worry about if it gets driven and used -and loses money.
An original engine that has been rebuilt may or may not hurt value. I don't know how the car was driven after the rebuild unless is has zero miles on it and has a guarantee to back that up. Who rebuilt the engine, what extent was it rebuilt, were factory replacement GM part numbered items used, were original parts kept and used, were aftermarket parts used, and I want to see receipts and a phone number where I can call the machine shop who rebuilt it. If Bob's engine shop is outta' business and I can no longer talk to him, I won't believe the engine was ever rebuilt, even if you do have engine receipts (which could have come from another Pontiac engine build). If your brother-in-law who has rebuilt Pontiac engines for 40 years rebuilt it, I don't know your brother-in-law. Does the rebuilt engine come with a guarantee, probably not, but if it lets go in 6,000 miles I'm going to be back to pay you a visit seeing I paid TOP DOLLAR for a car with the claim it was completely rebuilt. And an "as is" sale won't protect you(not with my disposition LOL). I would feel more comfortable if you provided me with an engine oil analysis (sent out to an independent company that does this), a compression reading on each cylinder, a leak down test to tell me the condition of the rings, a coolant test to check for pressure leaks and exhaust gases in the coolant, and an exhaust gas analysis to tell me the condition of the fuel delivery system. If you had all this in hand, I might be more inclined to believe you on the engine.
Personally, I'd rather have an original smoking, running rough, worn out engine that needs to be rebuilt and get it done so I know it was done and what went into it.
I prefer original paint, good or bad, so I can see what the body really looks like and can look for those bad spots that can be repaired/fixed under the professional paint job only to resurface in a couple years. A professional repaint is no longer original and to what extent was it professionally painted? What brand/type of paint was used? Same as the engine, who did it, give me a name, and show me receipts and the stage by stage re-paint photos.
I don't mean to sound harsh or defensive in my post, its just it is very easy to pull the wool over a buys eyes, especially when the buyer has the cash in hand, a romantic gleam in their eye of days gone by, and no clue as what to look for or expect. I have been to a ton of car shows, looked at a ton of cars for myself, work on them, worked in auto body shops, and see price tags coupled to words like clean original body -new paint, rare, no rust, big block Pontiac engine, 4-speed conversion using factory parts, etc., only to see the sanding marks/bondo through the new paint, rare (they made 75,000 of 'em), run if they say big block Pontiac, a 4-speed conversion may not be factory unless all numbers are known - especially if it was supposed to be a Muncie and you installed a Saginaw.
I think it is up to the buyer to know his cars, ask for the appropriate documents for in hand viewing, have the assorted engine analysis checks done, documented, and in hand, and be given an honest appraisal as to original, restored, partially restored and exactly what was replaced/restored, modified car and what was modified/changed, or flat out UNKNOWN. The same goes for the seller if he really wants to fit the car to a buyer.
In my opinion, some upgrades/modifications to an original car can actually be good selling points, such as Vintage Air was added, disc brake conversion, sway bars added, upgraded/rebuilt OD transmission installed, etc..
I think anyone who is looking to buy a GTO has an idea of what they want and can then vary from that ideal to accept some things that might not be exactly what was in mind. With all the reproduction pieces and aftermarket upgrades, a GTO buyer can almost personalize his/her purchase to become the car of their dreams whether it be original or not.