Buying an existing business is a more certain (although NOT guaranteed) way of starting a business. There should be a number of years of accounts showing how the business is going.
There are two fundamental issues (although not the only ones): cashflow and net profit. The accounts ought to show that the cash generated throught the year on a month by month basis is capable of meeting the bills as they arise during the financial year. If bills are coming form suppliers, etc. on a regular basis throughout th eyear but the business earns most of its revenue, and so generates most of its cash, during only part of the year ask yourself the question 'How am I going to meet the bills?'

The second issue, net profit, isn't as simple as it seems. profit on its own is meaningless; it is the profit you expect in comparison to how much capital you are going to use in the business. This is known as the return on capital employed (ROCE).
The capital used in your business idea could be used in a different business or even simply deposited and left to earn interest. All financial ventures carry some risk. Money deposited in a bank runs the risk that interest rates will fall and with it the returns earned. However, there is no doubt that investment in a business proposal is the riskiest of all. This means the returns have to compensate for the risk.
A good rule of thumb is that the earnings after tax should be around 20% of the capital employed. That is, the business will pay back the investment in 5 years. More ....