I'm sorry to be the breaker of bad news, but the best you'll be able to do is a Toyota Avalon. Every one of your requirements are contradictory. Good gas mileage???heavy. Reliable, durable, and under $6000???example is a 2000-2005 Mercedes C class, which is not going to last. The Avalon will last, but the ones that are $6000 have more than 150000 miles. Also, a heavy car doesn't get good gas mileage. The Avalon doesn't. Look for a 2005 or newer Avalon.