Freedoom is an IWAD that's meant to replace all of the assets in the original game files with free versions. You can then use it to play any addon level, like the massive total conversions or level packs that win awards every year. A lot of modern level packs use a free texture set called otex that's a massive/near complete replacement of doom textures too. Sure you won't be fighting exactly the same monsters but Freedoom is surprisingly high quality and worth checking out.

And ultimately the original games go on sale for digital purchase for a few dollars all the time so they're not a huge expense to eventually get and then play all the free levels people create as they intended.