The functionality of the server edition is essentially the same as the free desktop version (other than multi user access)
So install the desktop version and see if that achieves the functionality you need.
Manager is a commercial product. No you can’t have the source code.
It is customisable, see
- Custom fields. See Guides → Customizing a Business → Custom Fields
- Custom reports. See Create custom reports | Manager
- Batch operations. See Use Batch Create and Batch Update functions | Manager
- Custom themes. See Guides → Customizing a Business → Themes
- Localisation. See https://forum.manager.io/c/localizations/82 & Localizations - New way to contribute
- Manager API. See Search results for 'api' - Manager Forum