In 1992, when David Gooding worked in the car department at Christie's, he received a telephone call from a lawyer charged with settling the estate of an elderly woman who had died alone in Vevey, Switzerland. For a time this woman summered at a hotel in Greenwich, Connecticut, and among her possessions there was a dusty old Mercedes that had been stashed in a warehouse, surrounded by old carnival equipment. "It's black," Gooding recalled the lawyer saying. "It's a convertible, and it's old."

"How old?" Gooding asked.

"I don't know. But it has pipes coming out of the side."

This was no ordinary Mercedes. It turned out to be an exceedingly rare 1936 540K Special Roadster. Fewer than 10 such cars are said to survive in comparable condition, and Gooding, who is now the president of his own auction house, Gooding and Company, believes the completely restored roadster might set the record for the most expensive car ever sold at auction when his firm offloads it this month during the Pebble Beach Concours d'Elegance. "We sold a Ferrari Testarossa last year for US$16 million," Gooding said, noting the last time he set a record. "If this car hits that number, it wouldn't surprise me."

Back in the early 1990s, Gooding failed to get the Mercedes consigned to Christie's, and eventually a private buyer swooped in. But now that same buyer is keen on selling, which means Gooding finally has his chance to bring the iconic roadster to the open market. "I've been chasing this car for a long time."

Much of the car's value is derived from its rarity and its condition - "every panel on this car is original," Gooding said It also came with a colourful provenance. It was purchased 76 years ago by the family of a Prussian baroness who might have been a character from an Alan Furst novel: she was elegant, coquettish, depressive, a dead ringer for Marlene Dietrich, who fancied jewels and lived in the best hotels in Paris and Cannes.

This was the Baroness Gisela Josephine von Krieger. She was a society fixture in London, even attending the coronation of King George VI. She resided in France until 1938 and, after being summoned back to Nazi Germany, fled to Monaco, then Switzerland. The Mercedes, which was being repaired when war broke out, was shipped to her after she negotiated a Swiss visa by feigning an illness.

In the 1950s, von Krieger brought the roadster to New York, where she lived in hotels like the Waldorf-Astoria and the St. Regis. She left the car in the US when she returned to Switzerland in 1959. In subsequent decades she grew increasingly reclusive. In 1989, when she died of a heart attack, she was living in Havishamian squalor, with US$320,000 worth of jewels by Cartier and others - later auctioned off by Sotheby's - scattered around her filthy apartment. "She died alone. It was a tragic story," Gooding said. "When they discovered her body, she had been dead for some time." Her Mercedes Roadster, secreted away for four decades in Connecticut, proved to be the best custodian of von Krieger's glamorous legacy. When the car was found, the glove box contained a pair of the baroness' white silk gloves. In the ashtray were ancient cigarette butts, stained with lipstick.