After the rock legend died in 1991, his former fiancé and lifelong friend Mary Austin kept everything he owned enshrined and largely unseen in his London home. On Wednesday, Ms. Austin said she was finally ready to sell off his belongings at Sotheby’s this fall.

The London auction house said it will sell his Estate during a weeklong series of sales starting Sept. 6.

“It was important to me to do this in a way that I felt Freddie would have loved,” said Ms. Austin, who stewards his estate, in a statement, “and there was nothing he loved more than an auction.”

The singer started turning up at auctions soon after Queen found fame as a progressive rock band in the early 1970s. Sotheby’s said its specialists often stayed late so he could come by and preview offerings on his own, and he either returned to bid on pieces or sent Ms. Austin to bid on his behalf.

“He was a cultural magpie,” said Oliver Barker, chairman of Sotheby’s Europe, “and he was a great shopper, going to jewelry and glassware shops wherever he traveled.”

The resulting estate of roughly 1,500 pieces encapsulates Mr. Mercury’s taste for finely drawn works on paper and delicate objects spanning 30 collecting categories, but it lacks the edgy, wall-power paintings preferred by billionaire trophy hunters today, Mr. Barker said.

Overall, the Mercury estate is expected to sell for at least $7.4 million, with prices sweeping from an estimated $500 for his tiny Tiffany mustache comb to an estimated $500,000 for James Jacques Tissot’s portrait of his mistress, “Type of Beauty,” the last piece of art the singer ever bought, the house said.

These estimates pale when compared with recent sales of other iconic musicians’ collections, from David Bowie’s $41.1 million estate sale in 2016 to Elton John’s $8.2 million sale of cast-off costumes and memorabilia in 1988.

It remains to be seen if memorabilia collectors will go gaga for Mr. Mercury’s hord of rarely seen guitars, glam-rock costumes and sheafs of handwritten Lyrics in the sale. Offerings will include his original lyrics to 1970s hit songs “Killer Queen” and “We Are the Champions.” Sotheby’s expects to sell Mr. Mercury’s one-page draft of the lyrics to “Killer Queen,” which comes accompanied by four pages of music notes and chords written on Elektra Records stationery, for at least $62,000.

The house said it will ask nearly $250,000 for his handwritten manuscript working lyrics to “We Are the Champions,” which was released in late 1977 and quickly became a sports stadium anthem.

Currently, the record for any popular music manuscript is held by Bob Dylan’s $2 million original lyrics for 1965’s “Like a Rolling Stone.”

Mr. Mercury’s auction appeal remains largely untested, but collectors got a hint last fall when RM Sotheby’s in London sold a 1974 Rolls-Royce Silver Shadow that had once belonged to the singer for roughly $355,000, nearly 10 times its high estimate. (The singer had to be driven because he never got a driver’s license.)

Mr. Barker said the house plans to turn its entire London building into an immersive exhibit about the singer’s career and collection. The exhibit will re-create several rooms of Mr. Mercury’s home in West London, known as Garden Lodge. It will open Aug. 4 in Sotheby’s London location and run until Sept. 5, which would have been the singer’s 77th birthday. Smaller touring exhibitions will also be sent to New York, Los Angeles and Hong Kong.

Fashion will play an outsize role in the exhibit and subsequent sale, including the singer’s velvety red crown and cloak worn during his finale rendition of “God Save the Queen” during his last tour with the band in 1986. The set will carry an estimate of roughly $74,000.

The sale will also include a military-style black silk jacket that Mr. Mercury wore to his 39th birthday party in 1985; it’s estimated to sell for $12,400 or more. Another piece to watch: The cat-covered waistcoat he wore in his last music video in 1991, “These Are the Days of Our Lives.” It’s estimated to sell for at least $6,200.

Mr. Mercury particularly admired Japan; the estate sale will include his collection of kimonos as well as vintage woodblock prints such as Utagawa Hiroshige’s “Sudden Shower over the Shin-Ohashi Bridge and Atake.” After Ms. Austin tried to buy a version of the 1857 print at Sotheby’s but got outbid, the singer found and bought another version during a trip to Japan. His version of “Sudden Shower” is estimated to sell for at least $37,000.

Tim Rice, the “Aladdin” lyricist who helped write two songs on Mr. Mercury’s 1988 “Barcelona” album, visited the rock star at Garden Lodge a few times over the years. Although it was situated on a bustling block, Mr. Rice said the home sat ensconced behind brick walls and a garden. “As soon as you stepped in, you felt like you were in a country home,” Mr. Rice said.

To Mr. Rice, the juxtaposition extended to the collection within, adding, “Freddie was a friendly chap and he liked people, but he also needed to get away from it all.”