Since the business is very simple and volume is small. Authorization is not required, real-time alert is not must at the moment as well. So I am looking forward using free license for a while. Is it fine? I am not sure what means "Splunk Free is designed for personal, ad-hoc search and visualization of IT data"
The license allows you to use the Free version in a business environment (subject of course to the rest of the license terms). This license may be displayed on install or first time run, or can be found in a text file inside the installation folder.