Yeah for any type of extensive grinding, it is better to talk to your GT5 friends and have them share the car(s) that you want and you do that same. If everyone shares a bunch of their good cars, there is no need to put wear on cars solely for the purpose of grinding.
I've been using someone's X2010 carbon prototype car for the 9 hr & one of the 24hour endurances, after i read it don't add mileage to the car. even tho i now have my own i thought it would be better since there is no wear on either car doing it this way
By damage do you mean engine and chassis wear? If so, then yes, you get the car they uploaded to the internet for sharing, as it was when they uploaded it.
If you mean damage as in wrecking in a race, then no... that's 'repaired'.