Hi Runelich
I just started to make a module, but I can give some help.
1. You can make your own grafics, maps, sprites etc. So not to many limitations. I did some modules for Solasta and the limitations there have always annoyed me a bit. In the other hand: Doing all by yourself needs A LOT of time!
You cannot (or I don't know how) put animated videos sequences in.
Making Sprites is a lot of work, but if you have access to premade sprites (I use some PVGames sprites, but mostly use the ones who come with the original module) you can use them too.
2. Probably the best for starters is the guide who comes with the Archmage DLC. And then, learning by doing

Sadly, it is hard to find information about specific topics in the net. But you can allway ask here in the forum.
Another good help is, open the scripting-files from the original module. There you can see and learn, how things are done.
You need a grafic program who can write pgn files (I use krita, but photoshop or gimp etc. will work too. Anything who can work with layers will do). All maps have to be in a grid of 64x64px, that means if you have a grafic of 1050x650, you have to scale it to 1024 x 640.
3. Yes, they do.
4. Yes, thats possible