The style of any house is partly determined by the proportion of solid area to "lights," or windows, and the style in which they are framed; any replacements must respect this balance even if the form details are changed.
A budget for replacement windows should be realistic and affordable, and include a cash reserve (depending on the size of the job and age of the building) for miscellaneous carpentry or structural repair costs.
In addition to new windows, the homeowner will need lumber for fill or repair, non-corrosive nails and wood screws, wood shims and assorted hand tools, including carpenter's square and level.
After the old windows are removed, window openings may need to be modified to fit the new windows by adding wood to jambs or closing in areas for smaller windows.
Rotted sills, jambs and previous remodeling muddles hidden behind trim or window frames may add time and expense.
Replacements should "fit in" with the general type of building in the neighborhood, being neither so cheap nor so expensive that they look odd: it's difficult to re-coup the investment when the house is sold.
Historic style, building codes and community standards may each dictate conventions about replacement