Nearly 50 percent of the condominiums inside the Boston Machine Lofts building at 13 Willow St. are sold, and more potential buyers are heading to the downtown address now that the model unit is completed.

According to matt Picarsic, the project manager, purchase-and-sale agreements have been signed for 12 of the 30 units.

"We're expecting to sign No. 13 this week, and word of mouth is spreading fast," he said Tuesday, noting that the new tenants include single urban professionals, young couples and other more eclectic investors looking for affordable home ownership rather than paying rent.

"A number of them already work in Boston and live in places like Needham where they're paying rent that, in some instances, is what they'll be paying to own a unit. Some that drive into Boston are now looking at the train because the station is so close by."

Picarsic said the development company, RCG Properties, plans to show the model unit to city officials and business leaders Jan. 20.

"We'll be showing the model unit so that people can get a sense of what the lofts will look like," he said, explaining that the units range in price from $158,000 to $300,000.

Twenty-five of the units are designed with two-bedrooms, the remaining five have a single bedroom. All have oak floors, exposed brick and fifth floor units are graced with skylights.

"People are really excited about it," Picarsic said. "They can move into a great space and everything is brand new except for the exterior of the building and that's beautiful as well."

Picarsic called attention to the oval granite-framed window that faces the street and floor-to-ceiling windows on some of the lower-floor units. Khalsa Group is the project architect.

"The early buyers can customize the finishes, opting for more light fixtures, granite counters instead of laminate, high-grade wood on the kitchen cabinets and things like stainless-steel appliances," he said. "Most of these upgrades are pre-priced, so it's easy to do, but the closer we get to completing the units, the harder it will be to order customized finishes."

The average unit costs $200,000. The condos vary in size, from 850- to 1,500-square feet. The rear units have windows on three sides. All have central air conditioning, 12-to-15-foot ceilings, and ample basement storage. Tache Real Estate is the broker.

"The city has promised us a certain number of spaces in the nearby municipal lot once we bring in the buyers," he said, noting that Parking Director Jay Fenton has been assisting the developers with plans to lease the parking spaces to the tenants.

Picarsic praised the city's installation of ornate street lighting in Central Square and the downtown business district.

"We've already talked to our contractor about matching the lampposts that the city installed, but putting in three acorn lamps at the building," he said.