There are absolutely issues with using brand names or designs - it's infringement. Some car manufacturers do not want their automobiles in games and will not license it - others would require a huge fee.
You can always rename the car. Thats what street challenge did, nitto legends, well they got the approval
Yes, just look at the Grand Theft Auto series, which 'made up' names of many popular cars because they couldn't use them without going through licensing.