Running DOOM and DOOM 2 with DOS

I did a bit of custom WAD writing back in the day, and from what I remember there were only two big rules when creating your own custom WADs:

1. You couldn't include any iD copyright material in your WAD: ie, no iD created sounds or graphics could be embedded inside your downloaded WAD (but you could link to the iD files in the original iD iWADs)
2. Your WAD could only work with the full, commercial version of DOOM. You could not create a WAD that worked with the shareware DOOM version. And as a WAD writer the full commercial DOOM WAD was much better to link to as it had a much wider selection of textures, sound, etc than the shareware one anyway.

So, if the WADs were written by anyone who followed iD's rules, you won't be able to use them with your shareware version. But, come on, you can pick up the full version of DOOM or DOOM II for pennies these days (either new on Steam, or for about the same price second hand elsewhere).

Of course now that the DOOM engine has been released for free under the GPL you could theoretically run any WAD with that, but if the WAD you download is a pWAD (patch WAD) that attaches to the (still iD copyrighted) iWAD files as a patch (as many of them do) then you will need to buy a legitimate copy of the iD WAD to use with whichever DOOM engine you have. However for total conversion iWADs that replace all graphics and sounds and don't use any iD copyrighted material (like the original Star Wars DOOM or Aliens DDOM) then you could use any of the free DOOM engines available now.

The difficulty jumps quite often rely on the map you are playing on and vary quite a bit between maps. Here is a general guideline of the difficulty levels you should expect. The information is taken from the Doom wiki. For difficult level on individual maps, it's best to consult their corresponding wikia pages.

Skill Level             Monster numbers   Ammo amount   Other
I'm too young to die    Least             Double        Player takes half damage
Hey, not too rough      Least             Normal
Hurt me plenty          Default number    Normal
Ultra-Violence          Most              Normal
Nightmare!              Most              Double        Monsters respawn, faster attacks, no cheats allowed

Source: Doom wiki.