The only reasonable basis for valuing a company is based on EBITDA (earnings before tax, depreciation & amortisation). However for a small company, such as that you are intereseted in investing, this basis would be the subject of too many variables to present an appropriate value. I.e. who would buy the company?
I would only consider investing in a company if I was very very confident with the current owners/management and trusted then fully with the level of investment.