In general: neither. The advice is to have the full source tree in /opt/ledgersmb/<version>.
Is there a place where we could buy some consulting for LedgerSMB?
or a minimal setup according to our specs?
Take a look at Commercial Support
Yes, its open source. The tools we use (Perl, PostsgreSQL, Apache ++) are also open source.
Open source means that the source code of the software is available for free to everyone and you can modify the code and distribute it yourself. https://en.wikipedia.org/wiki/Open_source
Look for a place to contribute. This means not only programming, but also documentation, theme design, art work, or architectural design.
Quite a few users find some of the terminology and accounting processes a little confusing at first. Some of the more popular small business accounting packages tend to hide these aspects of book-keeping from users for simplicity, so with the current user interface there is often some new ideas to grasp.
First step is the manual - the LedgerSMB manual is free and highly recommended reading.
The table below lists the compatibility of LedgerSMB versions with PostgreSQL versions. Products for which support has ceased due to End-of-Life date being reached are not listed and should not be used.