A proposed large apartment complex off Prospect Street that has been the subject of much controversy is hitting a crucial juncture. The Zoning Board of Appeals met last week to discuss conditions for the Simon Hill development and will make a decision on the 126-unit proposal at its meeting on Wednesday. “This has been a very complicated — and controversial — application,” said Lois Barbour, chairwoman of the board. “If developed as proposed, it will have a huge visual impact on the neighbors.” Abutters and other local residents have attended hearings to express concerns, which include density, noise, and traffic, about the Chapter 40B proposal in which 25 percent of the units will be affordable. The original plan called for 80 townhouses spread throughout the nearly 30-acre site. The zoning board has stipulated that development is only allowed on 17.5 acres, on the lower portion of the parcel.