An entry-level Claas Jaguar 820 forager retails for the wrong side of £100,000. Just over half that sum could secure you a more powerful second-hand model with a maize header thrown in. Sounds attractive, doesn’t it? But if you do decide to buy into used 800-series performance, you need to know what to look out for. Getting it wrong could cost a fortune. James de Havilland investigates