Solid growth looks assured for the next few years, but what happens after that is questionable.Whether the company is a buy is an interesting question, because it strikes at the heart of the bull-and-bear debate over Lockheed Martin (LMT 1.19%) stock. Here's how long-term investors should think about investing in the defense contractor.Buying Lockheed Martin stockInvestors often like buying stock in defense contractors because of the surety of their end market customers; after all, you can't get a more reli ...