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I heard that this game is very hard. Is that true? I am no newbie to such games and I don't fear missions that take hours, but I just wanted to know, if the missions are actually hard to win?

My second question is about the expansion TPR: What features does it add to the original game? More missions? Or also more units and buildings?
Now that I own the game I can actually answer the questions to myself. For any other person that is interested:

Question 1: No, the game is not that hard. Many missions just need some time to complete. However, you should save the game regularly, so if you mess up you don't have to start every mission right from the beginning.

Question 2: TPR (The Peasants Rebellion) adds a second campaign with 14 missions and it adds one building to the original game (fish can be fetched now).
i can make an add in for question 2

apart from fish , food source,
you get the town hall
a building where you can recruit 5-6 new military unit with gold. yes no need for production chain
so the barbarian you meet in the first game you can recruit now.
though they cost 5 gold per unit so you need an exstensive gold production for itthough a nice addon to help getting the last few units a little faster if you need them etc.

you got cheaper unit there to

other then that i can't remember though i can recommend the kam_remake with a lot of thing going on dynamic scripting system custom campaigns altso it adds a new campgain with it with 10 more missiong + support for widescreen and modern resolution and multiplayer lobby etc. there is actually a lot of player in the lobby to last i check there where about 20-30 active games there
Actually, the game gets progressively harder.
And it takes more time.

It it recommended to get the KaM Remake (unofficial fan made)
It supports 1920x1080, fully working multiplayer, better AI and fixed issues in the original