Should I buy an existing business or start from scratch?

Be very careful here. Why is the owner selling in the first place? Look at the books carefully. The glittering figures might have been cooked. Has it got a track record? Do you have fresh ideas to inject into the  business.

If you feel you must go through with it, engage a corporate lawyer to make sure you are indemnified against any existing liabilities. In other words you don't inherit people's debts.

Go further than this and do some detective work. Talk to an employee (preferably, ex) for inside information about the business.