Coventry Health to Buy First Health  CHICAGO (Reuters) - Coventry Health Care Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=CVH.N target=/stocks/quickinfo/fullquote"&gt;CVH.N&lt;/A&gt; on  Thursday said it agreed to buy rival health plan First Health  Group Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=FHCC.O target=/stocks/quickinfo/fullquote"&gt;FHCC.O&lt;/A&gt; for about \$1.8 billion in stock and cash  as it looks to expand its geographic reach and keep pace with  rivals.