The Martha’s Vineyard Museum expects to sign within days a purchase and sales agreement to buy the former Marine Hospital in Vineyard Haven for an undisclosed price.

The purchase of the historic property, perched on a hill above the harbor, would mark a major step in the museum’s long quest to find a new home for its historical collections outside of Edgartown.

Museum executive director David Nathans said yesterday morning he hoped to have a deal completed by week’s end.

“We don’t have any signed purchase and sale agreement, as of now,” said Mr. Nathans. “But we’re in the process and we’re close and hopeful. The indications are that we will hopefully get this to a happy conclusion from both perspectives, in the next few days.”

The museum began eying the grand old property, currently owned by the St. Pierre family, when it came on the market in April last year.

The 10,000-square-foot, 29-room main building dates to 1895 and offered medical care to soldiers and sailors and their families in peacetime, World War I and World War II. A substantial brick addition was made in 1938. The hospital sits on some 4.5 acres of land about a half mile from downtown Vineyard Haven.

The original asking price was $3.19 million, but Mr. Nathan indicated the museum would pay substantially less than that.

“I can’t tell you the price — the details are still being hammered out — but I can tell you I have not been authorized [by the board] to pay that much. It will be well south of the list price.”

Using funds provided by a private individual, the museum took an option on the property last year, effectively taking it off the market while the feasibility of the project was investigated. In November, the museum engaged the Vineyard’s South Mountain Company and Boston architects Conrad Ello and Matt Oudens, to come up with preliminary designs.

The resulting plan calls for the demolition of the 1938 addition, the restoration of the 1895 building and the construction of additional exhibition space in a new 7,500-square-foot west wing.

As well as allowing time for planning, Mr. Nathans said, the option to buy gave the museum time to seek further funding.

“We now have a number of big donors. We have more than enough money at this point to purchase the land and get things going [and] we have indications from a number of individuals who have been generous to this Island, that we should come back to them,” Mr. Nathans said.

Having decided the project was both technically and financially feasible, the museum let the option lapse a week or so ago.

The museum will certainly have to go back to those donors, and find others as well, he said, “but the most important first step can be taken, which is that we will own the property.”

All told the project cost is expected to cost between $15 and $20 million.

“We know now we can do it; the next step is to think about how we do it — all at once or in phases,” Mr. Nathans said.

“Ideally we would do it all at once, but we will likely do it in phases, much as the YMCA, did it,” he added.

Thus the next few months will be devoted to working out how that might proceed.

“We just need to think through what’s most important, what’s going to excite the early donors, he said

Some things had to come first, including site development, what Mr. Nathans called “the outside stuff, driveways, parking lots, tree removal, planting, infrastructure, power, et cetera.

“That probably all has to be done in the first phase, even if you’re not doing all of the construction. You put in the infrastructure knowing that’s going to come at some point. Then you’ve got the renovation of the old building. The intent is to probably get rid of the brick building and bring back the original 1895 building. We couldn’t figure any way to use the existing brick building in any kind of cost-efficient way,” he said, adding: “So taking the 1938 building away and restoring the 1895 building might come to around $3 million. The new space that would be added in a couple of different buildings would be another $6 to $8 million.”

Formerly the Dukes County Historical Society, the museum has been centered on its School street campus in Edgartown for all of its 70 years.

In 2002 the museum purchased 10 acres in West Tisbury and unveiled a $25 to $35 million building plan for the up-Island site in 2006. But that plan was put on hold after the capital campaign foundered. The museum board since then also considered and rejected the idea of relocating to the nearby old Edgartown School.