You will need the .WAD files from the original Doom games. Those who own Doom on Steam will find those in their Steam folder.
Below is a thorough guide to set up Brutal Doom with all high resolution textures and an entirely redone heavy metal soundtrack. http://www.mattloveswriting.com/how-to-install-brutal-doom/
If you guys need more incentive, I'll just leave this here:
I love how much more punch the weapons have in this. The guns in Doom already felt nice and tight and now with the added screenshakes and other feedback effects they feel as good as in any modern shooter.
You can also kick toxic barrels into groups of enemies then explode the entire group with a single buckshot. This is my absolute favorite way to kill demons .
And since you can configure the blood spatters to never fade away, you end up with some extremely bloody levels.
This is the greatest 20th anniversary present Doom could've received.