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Yes, they are correct, and yes, you can use the article with the make of car:
There is a Rolls-Royce at the door
The Alfa Romeo is a great little car
Without the article, it would mean the maker of the vehicle - Rolls-Royce is no longer a British-owned company
Nissan have become a major international company
Similarly, if you refer to a specific Hilton, it will be "I will meet you in the Hilton Hotel on Park Lane".
"Is there a Hilton Hotel in Tokyo?"
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