Question: Is XWiki easy to install and to manage? Is there any aspect / criteria one should consider before looking for a server to install XWiki?

Answer: There is one thing you should think of before choosing server:  You will either need a server with swap-space or you will need to over provision the RAM space.  Because of the JIT-compiling performed by java, the memory will gradually fill up with rarely referenced code.

This means that if you want to install it on a VPS you should prefer one based on the xen hypervisor.

You will need 300 MB of RAM for the application server, add another 150MB of RAM if you are running the database on the same machine, add another 150MB if you are also running an open office server, and add 500 MB if you have no swap space (1GB if you want to be sure that the server will not crash due to lack of memory).