From a strictly financial viewpoint, unless a set is particularly collectible, the time and money spent on "completely restoring" it is unlikely to be recouped in a subsequent sale. It's sort of like trying to restore a 1970's automobile: you could sink $8,000 into it bringing its drivetrain, interior, and body up to showroom condition; yet in the end you would have a car that is worth around $4,500 on the collectible-automobile market in pristine condition.
Of course, if the time/money aspect is immaterial to you, it's always a great feeling to "make something old new again". And we are only the stewards of these objects, anyway- so maybe you could look at such a restoration as an endowment to whoever ends up as its next custodian.
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