Des Moines-based mechanical contractor Baker Group has paid $3.4 million for about 20 acres of land in Ankeny where it plans a $22 million office and manufacturing plant. Baker Group’s 1600 Corporate Woods LLC paid $1.7 million each to an entity in Rochester, Minn., and and another entity controlled by Knapp Properties Inc. and Denny Elwell Co., according to Polk County land records that were filed Thursday. The Ankeny City Council has approved an estimated $1 million in tax rebates for the project, which will involve relocating 480 jobs to Ankeny. Baker Group is expected to be the city’s third-largest employer after the project is completed.