* Great Universal Stores PLC said Friday that it plans to acquire an Illinois direct marketing firm in a deal valued at $831 million and merge it into Great Universal's Experian information company in Orange. Great Universal said Friday it agreed to buy Metromail Corp., of Lombard, and began a tender offer for its stock at $31.50 a share. Terms include assumption of debt. Metromail also operates a large mail service and publishes local business directories. Few, if any, of Experian's 1,200 employees in Orange County would be affected by a merger, said D. Van Skilling, Experian's chairman and chief executive.