Whilst the main lighthouse itself is owned and maintained by Peel Ports, the couple will be making over their whole house below the structure.
They will also turn the outbuildings, currently roofless relics, into a fully functional home including bedrooms, a kitchen and lounge area.
Captain Telfer, recently named a Freeman of The Honourable Company of Master Mariners, told the Tele: "Owning a property like this comes with a great responsibility.
"We chose Benjamin Tindall as he has great experience dealing with old and historic properties - he worked on Kelburn Castle in the past so understands the importance of the project.
"This was our holiday home, but now we're here permanently so we thought we would do a top to bottom renovation.
"Having this home is a privilege, so we want to make sure we do it properly."
One of the biggest changes the couple are making within the property is moving the kitchen to the back to overlook the Clyde.
A full wall-to-wall glass window on the back of the property will be installed and the roofs on the outbuildings will be restored to how they once were in the 1960s.
The couple say they have had some negative reaction on social media, so want to ensure locals that they are mindful of the history of the building.
Junn, who hails from Singapore, said: "We are excited about this project and want to show people who have been saying negative things on social media that the outside of the building will not change, except for the outbuildings.
"We are making this our home and changing the inside to suit."
The lighthouse, which was built in 1797, was acquired by Duncan in 1985, when he was 26.
He then went onto purchase the keepers' house and land surrounding the lighthouse in the years that followed.
The couple are expecting to have the work completed for February before they welcome Junn's family to Scotland for their wedding in May 2022.
The retired sea captain, who is 62, added: "We are very proud to live here and cannot wait to see it transformed.
"It's a huge project but it will be worth it to see it completed."