ePO 5.1 comes with SQL 2008. If you would like to install sql 2012 then please install it prior to ePO installation.
As a clarification, there is a SQL lite version that comes with ePO. If you are using ePO 5.1 in an enterprise you should install the full blown SQL version. If you are a small business of 200 systems or less, you could consider the lite version. There are hardware specifications so make sure that you grow into the server and not out of it.