If you're resourceful then you shouldn't have to buy it, but yes, the Steam version should be the same as the physical version.
Just wondering about something, when it comes to prboom (shareware doom) on the 3ds, I tried adding another WAD file just to see if the level would be available in the 'new game' selection, but it wouldn't show the level id inserted along with the other prboom wad files. :. ?
http://www.acolumbinesite.com/eric/doom.php