Assess risk! Nothing is guaranteed. If the risks are too great don't invest.
Appearing viable via the calculations below doesn't make a project viable in itself. There are other factors.
Percentage growth estimates must be realistic. Over optimism will likely lead to disappointment.
Be clear on the term. After '?' years the investment must return '£?'.
An asking price for a business greater than its value doesn't necessarily mean it is a poor investment. ROI matters more than pride.