My I've managed to find a PowerEdge T110 II server cheap, I have no experience with proprietary servers. I will probably run Linux, however the raid cards it ships with do not support it, I was wondering, Is it possible to add my own raid card that is compatible? Also is it possible for me to install a different version of Linux, or something like FreeNAS?
Presume it's a PERC H200. You should have zero problems with it and Linux as long as you initialise and create an array before trying to install anything. Yes you would be able to add your own RAID card if you preferred, you may need to change the cables though.
The only issue you would face is with ZFS and the fact that the H200 is a RAID card and not a HBA. So you would not be able to use ZFS.
Dell perc cards are generally lsi cards with custom firmware, look and see if there are users that have a guide on flashing it back. Even if you run hardware raid freenas still has its benefits, easy jail management and php virtual box alone make it a great platform.