Except on those rare occasions where the metalworking, soldering and other hardcore machinist activities are the main draw, the classic car hobbyist is more interested in working with the engine and the outer look of the vehicle. There's a reason you're drawn to a particular model, and it's the passion of seeing a beautiful machine with a purring engine that most of us have in mind when we begin.
That means that, for the layman, the only absolute requirements for a project car are that the frame and engine block be free enough from damage that success is possible. There's a reason it's called a total loss when the frame or undercarriage is bent beyond recognition, just like there's a reason your stomach bottoms out when you hear phrases like "cracked engine block." Cars are made of steel because steel is strong, not because it's easy to repair, and the kind of damage that can warp a frame or destroy an engine is the kind of damage most of us aren't interesting in fighting back from.
Likewise, panels on the outer frame of the car can be rusted to near-unrecognizable states that aren't always immediately apparent until you've really gotten inside the machine or taken it apart. Be on the lookout for these major losses, and remember that with automobiles, it's almost always worth looking a gift horse in the mouth. Sellers know a sucker when they see one, and it's up to you to protect your interests. After all, you're the only one who's going to be paying for repairs, insurance and more.
The other difficulty lies in discerning your natural, temporary frustrations from signs that the particular model or project is worth abandoning. Things will always look brighter in the morning, and, after all, you got into this project for the excitement of the hard work it's going to require. On the other hand, you can't let your dedication -- or just an inability to accept defeat -- lead you down the rabbit hole of misspent time and effort that might keep you working on a failed project long after you've lost your passion and your interest.
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