A four-decade-old apartment complex in Annandale will become Fairfax County’s latest and most expensive purchase in its program to preserve affordable housing, the county announced this week.

Housing officials plan to ink a $107.5 million deal to buy the 672-unit Wedgewood Apartments located off Heritage Drive, which it will hold at lower rent for families with low and moderate incomes. The purchase will be funded largely through the county’s allocation of one penny of its real estate tax to the affordable-housing program.