Can install a test machine with Oracle 11G R2, not express edition, on a VirtualBox?
Most of your questions are covered in official documentation.
http://docs.oracle.com/cd/E11882_01/nav/portal_11.htm

Read the installation guide (NOT the "quick installation" one).

I don't recommend using "pre-built" virtual machines - try to install OS and db from scratch. The process is the same as installing on "real" hardware. Use CentOS as mentioned in comments.

Not sure for RAC, but Data Guard works perfectly.

Good luck and have fun :)