Graham,
It is entirely possible and often happens that horsepower exceeds the weight x surface area x friction coefficient formula.
I do not suggest a replica over an original, that is a decision that you alone must make. I am saying that if the overall economics are a consideration, resale alone is not the full picture. I offer the following for consideration, not as a suggestion. Say you bought a used Diablo for $80,000 and drove it 10,000 miles over the next several years. Maintenance would run $60,000 making your total expense $140,000. And you are right, the only Lambo maintenance in South America is on the other side of the continent in Brazil. Now you sell the car, and giving you every benefit of the doubt, you get the full $80,000 original price. Your total overhead in the car would be $60,000. Now say you bought a new replica for $80,000 and drove it the same 10,000 miles, with a whopping $1,000 in maintenance for oil changes and a hose and/or belt or two. You have a total of $81,000 and again giving you every benefit of the doubt say the car lost half of it's value and you had to sell it for $40,000. Your total overhead in this car is $41,000 as opposed to $60,000.
They should all be certified for the street. When I talk about certification, I mean for the country.
I am interested in building you a car, and you have nailed the price at 60-70 for a car that we both would be proud of. I will PM you with further contact info.