Nearly 14 years late and $4 billion over budget, the Enrico Fermi II nuclear power plant may finally go into operation today. That means principal owner Detroit Edison can begin charging customers for the electricity the plant produces and for most of the construction cost. Edison has been paying $75 million a month in financing costs for the $4.58-billion plant. Fermi was licensed by the Nuclear Regulatory Commission in July, 1985, and testing was supposed to have taken six months. Barring a last-minute shutdown, it will have taken 2 1/2 years. Located about 40 miles south of Detroit along Lake Erie, Enrico Fermi II occupies the same site as a previous reactor.